Most of the interfaces you're discussing are more-or-less "purely functional", though. Interop overhead is going to be a lot less when you're squaring a matrix

Most of the interfaces you're discussing are more-or-less "purely functional", though. Interop overhead is going to be a lot less when you're squaring a matrix than when you're trying to write an event loop. In fact, most people who use those C++ libraries never touch C++, as you've pointed out yourself: they use Python or (recently) Julia. The ecosystem as it exists is already a hodgepodge of C[++] and Fortran, and while the interop between those might be annoying if you were building a video game, a NumPy user doesn't care if one subroutine is in C++ and the next one is in Fortran. So, I don't see why one more backend language would be a problem, realistically.