Batman has plenty of contingency plans, even those for the most absurd and unimaginable events.
Batman must be prepared in case some superhero is under mind control or if they turn to the dark side. After all, Earth is inhabited by some of the most powerful beings in the universe, so Batman was forced to devise contingency plans against every member of the Justice League and every other hero.
Unfortunately, Batman had to use those plans to take down his teammates, his almost-friends who were under mind control, more times than he would like to admit.
Fortunately, Bruce never found himself in the situation of being forced to eliminate them permanently once and for all.
He knew that he would succeed but that would not please him in the least.
Moreover, Batman is not the only backup plan to defeat an out-of-control Justice League.
Bruce has been preparing and training Dick for this eventual opportunity since his eldest son was Robin.
When there were other Robins, starting already with Jason, Bruce realized that the training Dick had received was too hard, too efficient, and too intense and that he had turned his son into a perfect machine.
Often, whenever he saw his firstborn's face, Bruce was aware of how he had transformed and molded his son to be the ultimate weapon, the perfect machine.
Reason why Bruce decreased the intensity of training to all the other Robins but that is another matter.
Now that Dick had become Nightwing, one of the most efficient and experienced heroes around, Bruce knew that his son was one of the very few people in the world capable of taking down the JLA.
But Batman also needed a contingency plan to bring himself down in case of mind control or worse.
Bruce knew that his obsession with fighting crime was unhealthy to say the least, so he would not be surprised if it drove him to madness.
One of Bruce's greatest fears was becoming a monster that he fought against on a regular basis.
So there was a need to create a contingency plan against himself.
Initially Bruce had thought of the Justice League but then he reconsidered when he knew that he would be able to take them down and use his contingency plans against them to defeat them or worse.
Instead, his contingency plan was Nightwing.
Nightwing was a better fighter than he was, an amazing strategist and improviser, and Bruce knew that Dick would not hesitate to take him down.
Even permanently, if keeping Batman alive meant endangering innocent lives and heroes.
Bruce was proud of Dick and how he became a better fighter and hero than he was.
Nightwing had surpassed Batman in almost every respect and Bruce was infinitely proud and proud of that.
NOTE: The fact that Nightwing knows how to take down the JLA is not headcanon but is canon. In comic Grayson #8 Dick must face not the Justice League but their equivalent in android form: Paragon. Paragon is a robot with the ability to copy any ability and superpower. Dick was fighting Paragon who had the abilities of the Justice League along with Agent 1, the Tiger King, and Richard instructs him on how best to fight to defeat the enemy. There is also a quote from Dick that stuck with me.
I've fought beside the Justice League. Trained to fight against them. They've got weaknesses. So will he.