SPA like React, Vue, Angular, Svelt, solid, should be not needed if HTML implemented these features. Or make JSX a standard again such as E4X.

I’m all for standardizing JSX, but I think we should be clear that its only similarity to E4X is syntax. The latter corresponds directly to the browser/HTML/XML DOM, whereas the former corresponds to nothing in particular except the semantics you might give it. I’d also be interested in a revival of E4X as a potential implementation of standardized JSX, but I’d be disappointed (and I think many JSX users would be too) if the standardization coupled the two completely.

All of the libraries you’ve listed offer features above and beyond what we’re talking about here. Plus, web components exist, they do most of it and yet the libraries still exist.

See: $ becoming querySelector.

Yes, many frameworks including svelte took heavy inspiration from various parts of the Web Components specs, including named slots. So, this is the original thing that svelte and others copied.

And that's not a coincidence; svelte chose to borrow that concept from Web Components.