Correct - SSR the way Next does it is basically the same as HTML+AJAX but simpler to reason about as an implementer and, if you use Typescript, type-checked and far less likely to be buggy.
Yes, I agree. The "minimum complexity" is way higher this way, even for simple things.
But my fear is that when things get complex in the frontend, at some point there is a peak point at which these tools become easier and the more traditional ways more difficult. And it is not easy for me to day where that peak point is.
Have you ever had to debug non-functional SSR in React or Next.js? I very much doubt SSR is simpler than plain HTML or PHP or AJAX.
After all, SSR is just rendering plain HTML. It doesn't have to be that complicated full of gotchas and quirks like it is in the React world.