> This isn't about state, is it? In a classic react app, if I need to display the name of a user, then I fetch it (from the server) once, and display it as many times as I need, in as many forms as I need. There's only server state.

No, there's two states here: the frontend state, and the backend state.

The name example is trivial, but in real applications, your state is complex and diverges. You need to constantly sync it, constantly validate it, and you need to do that forever. Most of your bugs will be here. The frontend will say the name is name, but actually it's new_name. How do you do fix that? Query the backend every so often? Maybe have the backend send push updates over a websocket? These are hard problems.

Ultimately, there is only one "true" state, the backend state, but this isn't the state your users see. You can see this in every web app today, even multi-billion dollars ones. I put in some search criteria, maybe a check a few boxes. Refresh? All gone. 90% of the time I'm not even on the same page. Push the back button? Your guess is as good as mine where I go. The backend thinks I'm one place, but the frontend clearly disagrees.

SSR was so simple because it FORCES the sync points. It makes them explicit and unavoidable. With a SPA, YOU have to make the sync points. And keep them in sync and perfect forever. Otherwise your app will just be buggy, and they usually are.