If my argument were that the new version of Gmail was better than the old one, and if the old version being "far superior" had some objective grounding—then you might be on to something.

But that wasn't my argument at all. Gmail isn't even written with React. And if it were it would still be irrelevant: I'm not arguing "writing a program in React means it will be a good and well-designed program"; I'm saying if you write the same complex program twice, once with built-in browser APIs and once with React, you will on average find that the React implementation is far quicker to write, more concise, fewer bugs, and so on.