In APIs you're talking to 1 server, calling 1 function, that returns 1 kind of thing at 1ce.
XML starts becoming useful when you increment those numbers - any of them.
People (including me) have been trained not to think in those ways. The web still isn't a web - it's merely a collection of computers with individual relationships.
The 90s internet had fleeting attempts at other paradigms. We should probably put serious effort into a second bite at that apple since by every measure our technology is at least 1000x better in the same way we've revisited neural networks
The short answer, as you answered it, is:
> there are 1,000 page books on XML alone.
The long answer: it was over-engineered for the intended use-case.
It's easier to fix under-engineered stuff (like JSON) than to fix over-engineered stuff (like XML).
> If you like XML, there is a strong argument to support JSX because it's a way to keep XML around!
Well, this is basically pivoting XML to a different use-case. It'll still be over-engineered unless you stick to JSX only, in which case it's not XML anymore, anyway.
Just to emphasize this: Why has XML been losing as the standard config format? Why did JSON win on web APIs? It's not because JSON is better, but because it's simpler and very-javascript-compatible.
If you like XML, there is a strong argument to support JSX because it's a way to keep XML around!