> If Preact truly was as feature complete as React, it would be used by everyone by now
That's a false logical conclusion.
Preact (and others, like Svelte and Solid) are not only "as feature complete as React", they don't need some of the features of React (hooks are unnecessary when you have proper reactivity) and they are better at certain features (like SSR).
People using or not using them has nothing to do with feature completeness.
Fair enough. The quoted statement doesn't hold outside the context of the argument that Preact has feature parity. You can build the same apps with Preact and those others, you just need to sacrifice other things.
Again, there's no such thing as "feature parity" because some (many?) of React features are not required by other frameworks.
E.g. you don't need React hooks because Preact has signals: https://preactjs.com/guide/v10/signals/
Does this mean that Preact doesn't have feature parity? For a very strict definition, no it doesn't. Does it mean you need to sacrifice anything? No.
Same goes for many other frameworks. In modern landscape when it comes to features and abilities React is actually quite a poor offering.