I would say any feature worth building as an extension should be a browser feature (like Edge and Firefox have brought ad/tracker blocking). An extension or two for critical functionality is fine if you really, really trust the source, but the default should be hostile to extensions.

But how would you differentiate between useful and gimmicky extensions? If you give users freedom, it always comes with a risk. For the average user, a system like Safari on iOS might be the best solution while more advanced users should have the option to install whatever they like.