> MP4 can do so much more

None of that has fallen by the wayside though? I have seen examples of all of those in the wild except for interactive menus and multiangle. Multiple video streams is incredibly rare to encounter but I have run into it a few times.

The argument in favor of flexibility (and jxl) is that if you optimize things for the "average" web user (as the essay seems to be suggesting) then fairly mundane usecases require you to start juggling formats, support becomes spotty, and things start breaking. It's nice to have generous limits within which you can be confident that things will "just work" for the end user. Even just on my own system I'd much rather use a single format rather than dealing with app x not supporting format y.

tl;dr jxl is the mp4 of image formats and that's exactly why I like it.

The only things I agree with the essay about are progressive decoding and decoding speed. Particularly the latter badly needs to be improved.