> Isn't this evidence of it having a relatively small surface area that frameworks and other tools can build on top of, much like html, css etc?

No? You can build on top of a large surface area, too? Of course, the funny thing is, even if you are right and React has a "relatively small surface area" (relative being the operative word,) what good is it when the creators of the framework say that you shouldn't use that surface area directly?

> What are some specific sites you've seen that adopted react and ended up worse as a result of it?

I don't think you're arguing in very good faith, but one recent example is the Indian grocery delivery service bigbasket. They switched from mostly-server-rendered to Next, and the new version was downright horrid for a while, and is only barely usable now. The most cruel part of the joke is that the next.js version is currently only shown to logged in users, so you can blissfully shop for a while, get to the cart and then be hit with the awful new UI.