In general Next.js has so many layers of abstraction that 99.9999% of projects don't need. And the ones that do are probably better off building a bespoke solution from lower level parts.
Next.js is easily the worst technology I've ever used.
I don't know much about the nextjs and whether it was open like sveltekit currently is.
To me, nextjs (I think) was always meant to favour vercel but sveltekit has a rich history of managing multiple adapters.
Now, that being said there are still some chances of a rugpull that might happen but if that ever happens, I am staying on the last sveltekit that worked with cf and other cloud providers.
Rich and Simon are incredibly important, but they're in it for Svelte and the community more so than a paycheck from Vercel. Tee has been doing most of the maintenance on SvelteKit currently funded by community donations. And this isn't counting other infrastructure like vite-plugin-svelte or the Svelte CLI which are entirely maintained by volunteers. I don't think Vercel funds a majority of the work on Svelte even if it might be close to it.