Cython is a niche language for writing perf-critical bits inside your Python codebase. It's like C for people who don't want to learn C. At least that's how I treated it, when I had to write some stuff to make some numpy ops faster.

Cython is not in any real sense a replacement for a modern data/ml stack.

True but it's really nice way to get the benefit of type checking in Python.

Just like you, I had started using Cython for performance but then realized that I can discard a bulk of type errors if I used for type checking.

The other benefit is that the Python library ecosystem stays available.