How does it split the community?
Even if controller users don't want to be matched against kbm players- crossplay will be increasing the the pool of controller users to play with.
Edit: Are we just assuming a matchmaking system will be binary - player chooses the opt-in pool or the opt-out pool - no interaction between pools.
Any system worth its weight would first try to create matches with players using similar control methods. Then pull in opt-in players as neccisary.
So basically the opt-in crowd would potentially have more people to play with. The PC crowd would have more people to play with. The opt-out crowd will Have the same amount of people to play with.
Your edit is the potential concern. Read my other posts, I'm not belaboring this