And then one day, Astral creates a new Python implementation in Rust or something that is way faster and all the rage, but does this particular thing different than CPython. Whoops, you can’t use that runtime, because you now have cursed parts in your codebase that produce nondeterministic behaviour you can’t really find a reason for.

and then all the serverless platforms will start using Astral's new rust-based runtime to reduce cold starts, and in theory it's identical, except half of packages now don't work and it's very hard to anticipate which ones will and will not and behold! You have achieved Deno

That's a bit what it felt like when I was learning Rust async.

I get it, but "ecosystems" of async runtimes have a pretty big cost.

If the python core team cared about not breaking things I wouldn't need to run my tests on all versions of python.

If I know anything about the Python community - that new runtime would simply never gain significant traction, due to the incompatibility.

Well, in my early days programming python I made a lot(!!) of code assuming non-concurrent execution, but some of that code will break in the future with GIL removal. Hopefully the Python devs keep these important changes as opt-ins.