The 135i will be BMW's best cost to performance product in a long time, it's going to be a "recruiter" to first time BMW buyers.
Those buyers are younger, potentially driving sport oriented Japanese cars and used to being sold big discs with multi-piston calipers with Brembo and whatever on them. It also seems to work well with Porsche.
Not going with what the market expects here would be a mistake. I'm glad they made this change instead of looking like a stubborn luddite to new buyers.