Can a startup fail because its product is not a SPA?

One thing I've learned along the way is: Don't listen to "advice" from failures. People who have only failed will do you nothing good because they don't know why they failed (at least until they keep trying and eventually manage to succeed). They think they do, but they don't. It's like asking people of the ancient world if the earth is round. Their reality doesn't include a rounded earth, so they will of course say it's flat, and they would be 100% confident that they're right.

Also even the people who succeeded don't really know why they succeeded. In fact, the more successful they are the more clear they will be about the fact that they don't know how they succeeded. They will say something along the lines of "we succeeded because of luck and persistence and sometimes a bit of clever ideas". If you don't believe me just find someone successful and ask that question, if you can.

"I failed because my site wasn't an SPA" was the stupidest thing I've ever heard. It's nothing more than a backward rationalization to make himself feel better so that he can deny the fact that he was simply incompetent in many aspects. That's why you don't take advice from these people.

I seriously doubt that his startup failed because the whole page was being refreshed. More likely, it was a multitude of reasons, one of which was a bad UI, which he then blamed on the full refresh rather than the real problems with it.

I'd suggest that a better question is "Can a startup fail because its product is an unnecessary SPA?". Users most definitely see differences between the two, mostly that a bad SPA is unusable where a bad multi-page website can still be useful.

Go with your gut instinct. Server rendered with JS and AJAX sprinkled where it makes sense is a much nicer user experience and can also be extremely powerful where it counts.