I think that they did it about as well as they could given the driving motivation behind the change. The semantics of existing syntax changed with Python 3: 'foo' is a string of ascii bytes in Python2, but a string of unicode characters in Python3. Treating a string as both ASCII and Unicode within the same project, depending on which codebase it originated in, seems like a recipe for disaster.

Java => Kotlin is an example of a migration that's relatively painless, by design, and lets you incrementally convert projects over, source file by source file. But the Kotlin designers intentionally left alone certain misfeatures of Java to make that happen. It's still UCS-2 rather than UTF-8, for example, and it syntactically papered over nulls rather than actually fixing them with a true Optional monad like more recent languages, and its coroutine/async/await semantics aren't as elegant as Go or ES6 or Python 3.6. Changing runtime semantics of an existing language ecosystem is hard.