This is a result of my friends and I being sad during band class. One of them said something like, "What if John begged the Doctor to save Sherlock, but the Doctor said it was a fixed point! BLUH BLUH BLUH!" And then feelings happened and a headcanon was created.
Yeah.
"Save him."
The Doctor stared at John Watson. Once a man so strong, he was now so broken. The Doctor had never seen anyone so utterly hopeless. John was staying in the flat, but he wasn't living there. He hadn't moved any of Sherlock's things, hadn't entered the bedroom to straighten up the mess left from Sherlock's last tirade through the flat, hadn't sat in Sherlock's chair or moved the violin from the coffee table. Everything was left as it had been, with John in the middle of it, unsure how to go on with his life.
"You know I can't-"
"You have a bloody time machine!" John exclaimed. He would have risen to his feet, but with his cane out of reach, he just didn't see the point.
"It's not that simple!" the Doctor insisted. "There are fixed points and-"
"And why would one man's death be a fixed point?"
"I don't control it." The Doctor looked at him sadly. He could hardly bear to see him like this.
"And what would happen," John demanded, "if you did save him?"
"Time would collapse," the Doctor replied. "Is one man really worth the end of the universe?"
John wanted to say yes, wanted to make the Doctor do whatever it took to get Sherlock back, but he wasn't that jaded. He sunk down further into his seat and buried his face in his hands. "No."
"I'm so sorry, John," the Doctor said, turning towards the door. "I have to go, I wish I didn't. I'll check in one you sometime soon." Without another word, though he cast John a remorseful glance, the Doctor left the flat. He made his way to the end of the street, where the TARDIS sat, waiting for him with a tall figure leaning against it.
"You didn't tell him?" Sherlock asked.
The Doctor took a deep breath. "You asked me not to."
"Thank you, Doctor," he said. The words didn't sound forced, they sounded entirely genuine.
"You should tell him, Sherlock." The detective was about to say something else, but the Doctor wouldn't let him. "He's dying, you realize that? Every moment you're gone, every moment he thinks you're dead, is like another death for him! Just tell him you're alive!"
"I can't do that." His face was blank, unfeeling. There was no hope, no happiness. No life.
The Doctor's eyes fell. "I see." He turned to the TARDIS, inserting the key but not opening it yet. He just needed an excuse to look somewhere else.
"Goodbye, Sherlock." He closed his eyes for a moment. "I hope you do the right thing, in the end."