Just cursory looking at Jax, it does feel like >95% of the "everyone excited about Jax" people would get >95% of the performance win for 5% of the increased complexity by using Cython instead...
I used TF, Torch and Jax to implement a monte carlo sampling thingy and Jax was by far the most readable and performant.
I think the Cython use-case is kinda subsumed by the pypy jit right?