You're basically suggesting implementing JS within HTML, which htmx exists on a technicality, but not sure that's something we should be pushing at all. We're just spreading JS problems to HTML which is a perfectly fine language.
<fragment href="site-menu.html"/>
I'm not sure why JS would be needed here? (i.e in the same way html <select> elements are interactive without needing "JS").
Going a little more out there, what about cross domain imports for easy web component consumption?
<fragment href="https://weather-widget.org?location=london" />
It used to be sorta possible with HTML Imports but that spec got dropped [1].
But yes, HTML was clearly designed for producing output markup; but in reality the people who want to generate markup want to handle nested data, if/else, and loops, because that is better than copy pasting a bunch of markup everywhere and occasionally fat-fingering something.