a) This only works for some applications and a very bad fit which does nothing for others. Namely, this only works if 1) you have few hot code locations 2) that code uses data structures which can be feasibly mogrified into native data structures with Python accessors 3) the granularity is low, meaning few invocations doing lots of work each. If any of these conditions aren't met, "native extensions invoked by Python threads" tends to be ineffective but maintenance-intensive.

b) Introducing native extensions means deployment and distribution becomes more difficult, and introduces a whole new and large class of issues and caveats into a project.

c) Native extensions are not written in Python. (Yes, Cython exists, no, it's usually not a good idea to write more than glue in it).