How many mainstream programming languages do you know having equivalent issues as Python 2.x to 3.x? Do you think it is reasonable to rewrite your code base on every programming language "major version update"? One thing is deprecation of features, because of not thinking an API properly back in the day, and another, involving an important rewrite in order to get your code base working.

My point was: I expect Python 4 avoiding the mistakes of Python 3 regarding breaking backwards compatibility. Just that.