You can already use numba backend for many cases, just set the global pytensor.config.mode=”NUMBA” and forbid PyTensor from ever attempting C stuff with pytensor.config.cxx=""
With these configs you can use PyMC from pip install without immediate drawbacks. However numba will sometimes be slower (special compile times), and not all operations are yet supported.
We don’t have an issue for switching the default backend but you can follow, if interested, our attempt run the whole PyTensor suite in the numba backend: Run whole test suite on numba backend by ricardoV94 · Pull Request #811 · pymc-devs/pytensor · GitHub
That will have to work before we switch the default.