But all it is is fancier fetch()s. It can't remotely approach the functionality of React, so it just shifts any complexity to different places.

Yes, it shifts complexity to the backend. Which will already have it anyway (because it has to), has better tools to manage it, you can use literally anything you want instead of being forced to use javascript, has (relatively) instant access to the authoritative source of truth, and isn't resource constrained (at least not the same way that the client is).

so "just shifts it somewhere else" might be underselling it a little

And waiting much longer than an instant for a million server requests every time a template changes on a page is not a good use of those resources.