> The ~100ms between submitting a form and getting error messages isn't that big of a hurdle, in my experience

Sure, it's the >100ms between the user entering the first value that might have an error and the user submitting the form that's a much bigger problem between validate-on-server-on-submit and validate-on-client-on-entry.

> But I don't agree React _benefits_ developer productivity. On the contrary, I think it (and its competitor frameworks) are responsible for a lot of wasted developer time and frustration.

After a not-very-long familiarization period, developers seem to be more productive with React (or probably Angular.) I know I am, and I've been using SPA frameworks (mostly React) for a short time after having done web lots of other ways for a very long time.