There are tradeoffs. Further increasing the barrier to entry for new web browsers benefits the entrenched players and hurts end users by yielding fewer alternatives.

The entrenched are the ones deciding the features. This is their goal

That's what people say of C++ too. Too many features makes it harder to learn a language and ramp up on codebases; they'll have different standards on what they use.

Developers can use whatever features they want, but users can only watch as their computer uses up more and more energy because it suddenly has to perform yet another build step that previously was done on the server.