A lot. You don't have to write in C, just use the C-API functions. pybind etc. introduce a whole new set of problems, with new version issues and decreased debug ability.
But CPython itself has poor development practices: For about 8 years those in the inner circle can modify anything and pose as experts while brutally squashing criticism.
After cpyext and cffi, this is the third attempt, largely driven by PyPy people, to get a C-API that people want to use.
If they succeed and keep the CPython "leaders" who ruined the development experience and social structure of CPython out of PyPy, PyPy might get interesting. If they don't keep them out, those "leaders" will merrily sink yet another project.