Maybe I just have a different working definition of these words. To me "mature" means "fully developed" and "polished" means "achieved a high level of refinement". To me, rewriting it all to introduce a major feature that fills in a longstanding hole in the language doesn't say "mature and polished". Because often times many bugs are introduced into a codebase on a major rewrite despite extensive test suites, especially at the interfaces between features. Typically people might prefer a mature codebase to one that's just been rewritten precisely because it hasn't been vetted over years. "mature rewrite" sounds like an oxymoron to me, and I guess no one else agrees but I find it strange. That is all.

The rewrite started in 2017.

Fears about refactoring introducing bugs are fine and valid - but after eight years, haven't really happened. Seems the extensive test suite did its job.

This isn't a case of Python 2 v 3. Packages weren't broken en masse. The API remained stable.

If anything, the rewrite has proved that it is mature. Because they could perform a refactor without breaking everyone's everyday.