Disclaimer: I have not watched Detective Conan in a while, so some things may have changed since I last watched it and my memory may be a little fuzzy on some parts. Many points also come from members of the CoAi Discord I am in, and their analysis has allowed me to gain more insight into the characters. I am also in no way attacking anyone for shipping something that I personally do not.
Detective Conan is a show that I grew up watching. It was one of the few anime that aired on Chinese television back when I was a kid, and before I knew anything about the plot, I thought it was just a story about a genius kid detective who solved mysteries with his older sister. But then I grew up and started to investigate the series a little more, and that's when I discovered a whole new world.
I grew up thinking Shinichi must go back to Ran one day because they love each other, and love conquers all, right? But then I discovered the character of Ai Haibara, and I was immediately drawn to her personality. I started to see how much I enjoyed Conan and Ai's interactions, and soon, I was in too deep. As someone who has been a long time CoAi shipper, I have a few of my own thoughts as to why their relationship makes more sense.
Shinichi Kudo and Ran Mouri — ah, the "it" couple of Detective Conan. Without a doubt, they'd end up together at the end of the series, drawing a happy conclusion to a long-running story. However, as many would agree with me, there are several flaws in their relationship under the writing of Gosho Aoyama, and these flaws would be fatal for a real relationship.
Despite being childhood friends, they don't have much in common. Shinichi is a detective who would not stop talking about Sherlock Holmes, while Ran is a normal high school girl who does not want anything to do with detective work. Shinichi's whole life revolves around solving cases, while Ran, even with her parents being greatly involved in such matters, simply has no interest in mysteries. Most relationships are built upon common interests, and without anything in common, what is there to talk about? During the first episode, Shinichi was happily talking about Sherlock Holmes, a topic he was truly interested about, when Ran interrupted him, extremely upset: "You talk about Holmes, Holmes, Holmes everyday! Don't you have anything else to talk about, you mystery nerd?" Ran does not appreciate Shinichi's work and his identity as a detective. In contrast, Kazuha, whose relationship with Heiji is rather similar to Ran's with Shinichi, once said that she "loves the look on Heiji's face as he figures out the truth the most." The stark contrast between their attitudes towards their love interests' careers is almost hilarious, and it really reinforces how Ran and Shinichi just have a lack of common interests that will slowly eat away at their relationship. Sure, it may not influence them as much when they're close friends, and once they start dating it may not be the worst thing early on, but as time goes on, it will for sure end badly.
I used to really dislike Ran as a character for all her crying and screaming and overly dramatic actions, and to a certain extent, I still cannot agree with some of the things she does (and honestly, she has, like, zero plot relevance). But after thinking about it more and me maturing over the years, I think she's absolutely allowed to cry. With her childhood friend gone out of nowhere and rumored to be dead, she has to carry on with her normal life wondering what exactly has happened to him. She knows he's not dead with the few phone calls she gets from him, but there's too much she doesn't know about what's happening, and it's the mysteries that are absolutely destroying her. I've become someone who cries really easily and can get really sensitive about things, so I think her crying is justified.
Ran Mouri is a character who really took much damage from the author's writing. She's characterized as sweet caring, patient, kind — literally every compliment you can find under the sun has been used on her in the way of sticking labels to her character by other characters. She's capable of taking care of herself and her father, doing all the chores and the cooking, and her life's had some hardships, but she's always been able to walk forward with a smile. But with the labeling of "angel" on her over and over again, it just seems fake — it's hypocritical with how much violence she uses, breaking telephone poles with a single punch. It's problematic how she seems to use both violence and her tears to control Shinichi's actions, even though she doesn't mean it in that way. Even though she's portrayed as a karate champion, she'll still wait for Shinichi to save her, and I think that's the problem with the author's writing — he can't decide which set of traits to give her, so he gives her all of them, and she makes us think that she's a damsel in distress when also feeding us information about how strong she is.
The whole damsel in distress thing, which is the basis of essentially all ShinRan interactions, gets old rather quickly, especially there is proof that Ran can protect herself rather well. The formula is repetitive: Shinichi yells "RAN!" at the top of his lungs and dashes to save her as she is held captive yet again, and it has become annoying, frankly, over the years. The best example of this ridiculousness is Movie 17, when Ran falls into the ocean. She thinks of Shinichi before drifting into unconsciousness, and on the boat, Conan thinks about Ran, and as the radars slowly lose Ran's position, Conan starts to panic, letting out the loudest yell of "RAN!" I've ever heard. Then Ran magically appeared, even though she was drowning, and she thinks to herself: "Oh, I think I heard Shinichi's voice from somewhere…" And then she got saved and everyone was happy again. It was almost hilarious to watch, if not absolutely infuriating.
The biggest problem regarding their relationship would be their lack of growth. Shinichi has been in love with Ran since preschool all the way to high school, and yet there's been almost no growth in their characters and their relationship over these ten years. Nobody's perfect in a relationship — you help each other treat those flaws and become better versions of yourself because of each other. However, Shinichi and Ran have been their same old selves, and they haven't affected each other in positive ways at all. In fact, they are each other's restrictors; Shinichi has this sort of need to become a hero in front of Ran, and Ran gets dragged into situations that she really doesn't want to be involved in. Shinichi has had growth as Conan, but when he is put back in his old life alongside Ran, he seems to revert to that overly proud asshole that he used to be.
Conan Edogawa and Ai Haibara, however, are different. Although I know that they will never get together because Gosho Aoyama treats Ran like his own daughter, I can't help but become more and more interested in their relationship. They are tied together by fate, and the growth of their characters and their relationship over time has been something that can be easily seen.
Ranging from mundane things such as school and their Detective Boys friends to thrilling cases and life-threatening situations involving the APTX4869 and the Black Organization, Conan and Ai have much in common and many topics to talk about. In ordinary situations, you see them tease each other and joke around many times, which is something they can only do because of how close they are with each other. With Ai, Conan can finally talk about Holmes and the cases that he is unable to tell Ran about, and with Conan, Ai doesn't have to pretend to be someone she's not and is able to voice all her true thoughts. They are both their truest selves with no pretending needed because they are each other's best listeners.
Their understanding and trust for each other is deeply rooted in the intertwining of their fates, and it's insane how well they know each other and how much they trust each other for two people that have only known each other for less than a year in canon, even more so when they started out as enemies when they first met. When Ai grabbed Conan's hand on the bus in episode 230 in fear of the presence of a Black Organization member, Conan understood her immediately and acted accordingly to avoid any suspicion — she didn't need any words to convey her thoughts, and he understood her almost instantly. In countless cases, Ai supplements Conan's deduction, showing that she understands his thought process and is able to reach the same conclusions. The bond of trust between the two are also incredibly strong, demonstrated in multiple cases where Conan entrusts people he cares about to Ai, knowing that she will be able to take good care of them — in Movie 12, Conan delivered a message to Ai with his knowledge of her and her past, and Ai says "Moreover, I believe in him… As his partner." They're willing to protect each other, risking their own lives to do so. As a relationship is based upon mutual trust and understanding, Conan and Ai already have a stronger foundation than Shinichi and Ran do.
In contrast to the angsty crying and cliche drama that make up most, if not all, of Shinichi and Ran's moments together, Conan and Ai's interactions are just… Something else, showing a deeper connection between the two than ShinRan's surface level romance. I will always remember that iconic bus scene in episode 231, where Conan risks his life to save Ai from the explosion and telling her "not to run away from her destiny," which is something that Ai takes to heart. Movie six The Phantom of Baker Street is my favorite movie, and it holds my favorite line from Detective Conan ever: as Ai "dies" in the game, she says, "Don't give up. Even without the helping characters, we have our Holmes — you. There is that much ability within you. There are no cases that Holmes can't solve, right?" It shows how much trust and understanding Ai has in Conan, believing in his abilities and using her "last moments" to motivate him.
Over time, we see how Conan and Ai grow and develop as individuals because of each other's presence, and the growth of the two characters as well as their relationship is something that stands in stark contrast with ShinRan. Shinichi and Ran's love is almost shallow, as they do nothing for each other, while Conan and Ai have this sort of mature relationship where they become better people because of each other. Conan values Ai's input and her choices when it comes to different situations, whereas Shinichi treats Ran as "his" without any questions asked and displays an unhealthy amount of jealousy, which is honestly a little toxic. Because of Ai's presence, Conan has become a little more down to earth and less proud, understanding that he cannot do everything, and with Conan, Ai has become more happier and warmer, seeing more meaning in life than she has ever before.
CoAi content in canon, however, has drastically declined over time, most likely because of how Gosho Aoyama himself has realized how much "destruction" Ai can bring about to his precious ShinRan. He did everything he could to prevent that destruction from happening by making Ai a little fangirl of Ryusuke Higo and drastically decreasing her screen time both in general and alongside Conan. Aoyama's writing of Ai infuriates me; I cannot imagine taking a perfectly written character and just destroying her completely for the benefit of another character. It's infuriating, it really is, but I hope that CoAi will live in our hearts as how they truly are, because nothing can change the fact that they are bound by the red strings of fate.