There's an argument that this proposal involves very little up-front breakage. This is true but disingenuous, because the breakage will still happen in the future unless changes are made and committed. The entire maintenance load should be considered, not just the immediate load.
I don't have much to say about breakage (and thus I've refrained from voting), as a maintainer of just 7 packages of which exactly zero are compatible with anything else than the latest GHC (well, no one complained, I'm rather lazy, and if it's any defense I can't even run GHC 7.10.1 on my Arch Linux box because of inappropriate versions of dynamically linked libraries), but a honest question: does this proposal entail anything else than replacing every "return" with "pure" and conditional compilation of "return = pure" in Monad instances? Best regards, Marcin Mrotek