You're not entirely wrong but pretty much everything you use from python is written in C++ anyway, so what's your point?
The big shift is Jupyter, but that's mainly for exploratory programming. If you already know what you're doing, there's no reason why C++ should be worse than Python for training. It's likely that most ML engineers do not have experience with C++.
I don't have that experience either but from what I've seen, C++ is very powerful, so once you subtract the jupyter, there's not really too much left.
BTW: You cannot use DOM/CSSOM from C++, the only API is JS, so your argument is theoretical.