Just about any reputable sources puts the combined market share of React, Vue, Angular 2+, and Solid well above 80%.
So I am not sure what "we" you are talking about.
I have no idea if that's accurate, but let's assume for a moment that it is.
- Vue supports JSX, but it is not the default.
- Angular does not support JSX.
- Svelte, which you neglected to mention, does not support JSX.
- Solid does indeed use JSX, as of course does React.
So two out of the five main SPA frameworks don't even support JSX, and another doesn't typically use it.
As I said, you may have stabilised on JSX, "we" have not.
JSX just feels weird.
I think he's saying that the Web is hypermedia plus Javascript, rather than just hypermedia. Which means, from the standpoint of the power of hierarchy, that it's Javascript, which also happens to use some hypermedia. Even if you use only a tiny bit of a Turing-complete language, Turing-completeness defines what your system actually can do.
This makes the Web fundamentally awkward, because most of the behavior is defined by the hypermedia part, and users have inconsistent expectations for what happens when you break the illusion.
HTMX works by extending the illusion. JSX works by breaking the illusion earlier rather than later.