Very very few people, in practice, write vanilla CSS. They use libraries and frameworks often, like Bootstrap. Sure, that might be "vanilla CSS", but you didn't write it, presumably because you didn't want to.

Look, after a certain point we have to call a spade a spade and acknowledge that CSS is just too cumbersome, too awkward, and too complicated for most developers and companies. How much CSS do you see that seems to be read-only? People just... tack on to the end of the style sheet, right?

The promise of re-usability is great, but is it actually happening? IME, no.