And as I said in reply to a sibling comment: Then you are effectively maintaining 2 codebases. Also, I'm not aware of any SPA framework (at least the big 3) that even offer an escape hatch to do something like that. Maybe with SSR and some special logic you could but it would be painful and ultimately not worth it.
Remix provides something close to what you are thinking, the application works perfectly without JavaScript but gets enhanced if it's enabled which is pretty nice.
Remix, along with the other frameworks/libraries you've mentioned, are very interesting. I've considered trying out SSR with Quasar (the Vue framework I work most in), though selling it to "business" is hard and I understand why, I can't bring myself to eat the cost (both time and real dollar cost) on my own projects. I do hope SSR continues to advance, though I have some trouble imagining a "free"/"seamless" fallback for no-js users and so other than initial paint I'm not sure how functional some sites will be.
I specifically called out "web apps" in my first comment as I do understand the value SSR brings to things like blogs, news, or other simple sites where JS is not needed, or where it can have a clean fallback. On the other hand, I write "apps" (sometimes deployed on phones via Quasar/Capacitor as well as on the web) and those get much more complicated. I'm not quite sure how modals, WYSIWYG, rich date pickers, etc translate for a no-js user. Simple navigation is easy enough to grasp but my understanding is that things like NextJS/NuxtJS are really just for first render/paint and then React/Vue take it from there. I could be behind the times on what's possible without JS and using SSR through. I just know the PHP codebase I also work in uses plenty of JS to be functional (not above and beyond, literally "table stakes" stuff).
Yeah as of now I think the SSR capabilities of NextJS and NuxtJS will serve mostly for the first paint, it will also allow a user to navigate between pages without running JavaScript (which a SPA wouldn't). I do have to agree though that at a certain point thinking further than this about non-js users becomes too cumbersome and not really worth it if your application is truly a web _app_ meaning very interactive and to the point it could be bundled as a desktop application.
I'd like to note that Remix does handle everything being tied to a single logic as far as my testing went, I love it. The idea is that basically all interaction is done with html forms (like in the old days) and Remix loads a React bundle that makes that run client side after the page has loaded. It's a very simple model that should work for most use cases, although I don't think it's suitable if you're truly developing a web _app_.
As with everything, balance is key. JavaScript is useful and more appropriate is some situations, and it's not in others. I do hope to see more progress with seamless SSR for SPAs though, I think it would make the internet a much better place.
