Why does the number of people using a UI make any difference to whether the UI does its job well or whether the underlying technologies are appropriate, though? Would any of the arguments for different ways of building UIs change materially if Facebook had a million active users instead of a billion?

I'm just saying that even with that model before, we never had tech with the reach or capability as we do now, which makes that interesting.

If it was a good model for building UIs in 1970 and it proved itself then, we should be taking advantage of that on the frontend as well, instead of ignoring it.

In that case, perhaps our views here aren't so different. I'd agree that React was a relatively rare qualitative step forward in the tools available in web development, precisely because it did bring some ideas and programming styles into the front-end JS community for probably the first time. I just disagree with the idea that this was unlike anything that had gone before, because essentially the same fundamental ideas and programming styles have been used elsewhere in the programming community for a very long time.