This is currently solved by tiers, but:
Different users need different stability expectations:

• Performance chasers (e.g. Cython), need the fastest way to do something. It doesn't matter that much if the API breaks (as long as packaging can give users a working binary for their interpreter).
• Debuggers/optimizers need deep integration in the internals, and introspection tools. Again, the API can break when the details change.
• I assume that a lot of users want low maintenance (but we don't hear about these users much). They want API and ABI that doesn't break, even if it's e.g. n× slower that it could be.

Of course, it's a spectrum.