[mkl]

Matlab has changed far more over the years than Python (e.g. case sensitivity).

Switching from Python 2 to 3 is not a rewrite in the slightest - very little needs to change. Python 3 has many additional features which don't affect existing code (e.g. a matrix multiplication operator, type annotations, underscores as separators in numeric literals, etc.).

No language can improve without changing, and programs can't take advantage of improvements without changing. You are free to use old versions of Python and Numpy forever: languages with perfect backwards compatibility only have it because they're no longer improving.