We’ve reached different personal opinions, so I’ll only answer a part of this post:
Yet, that’s what happens for most pure-Python libraries. Why should native extensions be different?
PEP 387 applies regardless of stable ABI: if something breaks without deprecation, it’s a bug in CPython (or an explicit exception).
FWIW, a hypothetical PyPI build service won’t help much with this issue. We might need a test service. And/or perhaps metadata that would allow pip to warn “this package wasn’t tested on this version of Python”.