The sad story is that Apple could deprecate Carbon all they want, but if Adobe, Microsoft, Quark and friends kept using it, Apple would need to oblige. Apple had no power, and these companies were basically "too big to fail" so to speak.

It's also not the first time Apple got bitten by this problem (although to be honest given the number of massive changes they carried in the last 10 years or so, they kind of were asking for it ^_^).