Modifying the Python object model

PyPy is not always feasible for large existing CPython applications because they rely on the CPython API for tons of C/C++ extensions. The pypy C API emulation layer, even when up to the task, kills performance and increases pypy's already high memory overhead.

YouTube investigated using pypy. The experiments concluded that it just wasn't going to be a win compute resource wise even before counting the additional "different runtime" long term maintenance burden it would require.

PyPy has a lot of good ideas. It is a great place to experiment with fun internals changes but isn't practical for many things not already written to use it from the start.