It was, but it also feels like there's now a very large gulf between React's original selling points and the mess we have today.

You definitely can do it (I've done it myself including SSR hydration) but it was a massive pain to do.

It’s a PITA to extract a stateful react component to a standalone piece of code that can be inserted in a random place (in another page, served via API etc.). Not sure about Svelte, but achieving this in React was unexpectedly hard/impossible in our use case.

It doesn't.