My sense of Vercel (mostly from working with NextJS) is that they are more interested in appearing to support an open source framework while making their product as difficult to interoperate with other technologies as possible in an attempt to lock users into their platform and hopefully pay for it.
Hey, I'm on the team at Vercel. What could we do better? Open to your feedback. Our platform integrates with 30+ frameworks ( https://vercel.com/docs/frameworks ), we directly fund the development of Next.js and Svelte, and we sponsor Nuxt, Astro, Solid, and more.
Decades of working in this industry have taught me to value interoperability, modularity and standardization. I love the idea of a framework that makes SSR easy, and the idea of static site generation, and a router that uses the pages approach where each file is a route is handy in some cases (though very clumsy in others), but all of those a different things and aren't really something I need a single monolithic framework for. I may want one or some or none of those things based on what project I'm working on. If NextJS makes it harder to pick and choose what I want to use, or makes some features contingent on using other features I don't really care about, I'm going to start looking really hard for an option that gives me more choices.