The C extensions shouldnt be cut out of Python. I get that PyPy is better but cutting out c sxtensions and rewriting them in pure python would hurt performance more.
* Supported on more than just CPython. I.e. you can use them on PyPy as well.
* The extensions are far easier to install, since one doesn't need both a full C compiler as well as all development headers for the library installed on the server where you install the extension.
I wanted to use ctypes to wrap a C library for a recent project (for the ease of installation and development that you mention) but had to give up when it turned out to be more than 10x slower than a wrapper written in Cython, and barely faster than doing a pure Python implementation of the C library itself.