> Python really needs to take the Typescript approach of "all valid Python4 is valid Python3

Great idea, but I'm not convinced that they learned anything from the Python 2 to 3 transition, so I wouldn't hold my breath.

If you want a language system without contempt for backward compatibility, you're probably better off with Java/C++/JavaScript/etc. (though using JS libraries is like building on quicksand.) Bit of a shame since I want to like Python/Rust/Swift/other modern-ish languages, but it turns out that formal language specifications were actually a pretty good idea. API stability is another.