Absolutely they could. And they should, because doing so will get you launched quickly.

Instead, todays shops all start out with javascript front ends and 40 layers of backend complexity to accomplish the same thing.

I'm not sure. The bar for UI has been raised significantly since Facebook launched and mobile has changed the game to such an extent that you need a presence on 3 platforms out of the door (iOS, Android & web) to even stand a chance.

Not entirely true for many apps. The app I work on does not need a working mobile site, because we don't think our consumers are going to use it on mobile.

However, we do very much care about our UI - and I think react at this point helps with that. But I don't think React complicates the stack too much. It may even make things easier, tbh - I don't have to test pages on backend tech, just endpoints.

React easier compared with what - imperative PHP4 at Facebook in 2004? I'd have to disagree. React state management is much heavier than returning HTML from an embedded PHP template.

I believe so. But, most startups like to use insane tech for fun, or something - though in my experience all it does is kill productivity.