The advantage for me is that it uses JSX. I like everything about Svelte except the weird semi-custom language, module format, and magical state management. Being able to produce similar (though probably still less optimized) products with plain JS functions which simply return a data structure, and which can easily render to other targets besides the browser, is fantastic.

Not except this is in React. So similar thing. Also Sapper as of recently is no longer a thing :).

Can you expand on why Sapper is no longer a thing?

SvelteKit replaces Sapper https://svelte.dev/blog/whats-the-deal-with-sveltekit

Since g4k already answered, I can only say, isn't it wonderful at the pace things are going. And we are no longer having weekly new framework thing.

Is Svelte compile-time? I don't think RSC are compile time, just server-time.

They mention SSG as a goal in the RFC, so they intend to support build time (which would be transpile time if you want to split hairs).