> A React class component from 10 years ago will still work in React today perfectly well,

This isn't true.

ComponentWillMount and related methods were deprecated and then removed.

Yes, they allowed an escape hatch of UNSAFE_ComponentWillMount etc, but that still requires work to make it compatible, so it definitely doesn't "[work] perfectly well".

Then again, React projects tend to use fast-moving things on top of React, like Redux or NextJS or CRA or whatever. Personally, I enjoy writing stuff in bare React since I like its model, but people tend to steer well clear of that, especially these days with compilers and bundlers and whatnot attached to the frameworks.

I agree, bundlers and bundler plugins have probably been the main thing people see with JS ecosystem and think is overly complicated.