"> I think all of ML being in Python is a colossal mistake that we'll pay for for years.

Market pressure. Early ML frameworks were in Lisp, then eventually Lua with Torch, but demand dictated the choice of Python because "it's simple" even if the result is cobbled together.

Lisp is arguably still the most suitable language for neural networks for a lot of reasons beyond the scope of this post, but the tooling is missing. I’m developing such a framework right now, though I have no illusions that many will adopt it. Python may not be elegant or efficient, but it's simple, and that's what people want.