I would like to think that in any sane project they would choose one of the namespaces and stick with them. Moreover, I think that as you deprecated the older namespaces, this problem would go away.

You could make the exact same argument with removing register_globals: half the project using it, while half the project using the superglobals. Clearly there'd be some pain, but I don't think it would be as bad as you're trying to make it sound.

I'm not really sure I follow the logic that if someone has an issue with the state of a programming language then they shouldn't be developing. How does that make any sense?