* Major breakages to existing code (which needs to be updated), docs (many of which can't be updated), and future code (coding with ifdefs is error prone).
Pros:
* A feeling of cleanliness.
I think there's an implicit argument being made here, that if we let time go towards infinity every breaking change is eventually paid off, no matter how small the gain. Time doesn't go to infinity for us. Haskell currently has a window of opportunity for being adopted and bringing more functional programmers to the world. This window isn't very big, perhaps a couple of years to a decade. If we make programming in Haskell annoying by continuously breaking anything, people will lose interest in Haskell and move on to other languages.