Is there any way to use Sapper.js with React, Vue or anything framework? I find the premise behind Sapper interesting, but I just don't see the appeal of Svelte, where they are basically reinventing HTML syntax.
We're not reinventing HTML, we're using HTML. And CSS. ('Reinventing HTML' is a far more fitting description of JSX, and then there's CSS-in-JS...)
Doing so is what allows us to statically analyse your app and compile it to a) a tiny client-side JS payload, and b) an incredibly efficient server-side renderer.