There are things that HTML could do, and should be doing, that is not done or not yet possible simply due to hype and trend from browser vendors. We could continue to polish HTML + sprinkle of Javascript to its absolute maximum before hitting JS Apps. Right now this is far from the case.
You could provide multiple image versions for zooming to get to the TB scale.
Computers are really good, performance is astonishing, no reason why we should never be able to use a TB size image. Never is a really long time.
Have epic panoramas, detailed scans from paintings, extremely easy game design and maps that just work.
At the end of the day it isn't really the tech that is the problem. Is how people use the tech. And for thousands of different reasons keeping it simple has always provided better experience evaluated on the whole.
Github's old frontend was mostly HTML with a bit of JS, their new frontend is react. The old UI had its bugs, but it was much better than the react version in my experience. I still commonly find the UI out of sync with itself requiring a reload, but now I also frequently wait for the page to load and viewing large diff's is a performance nightmare.
The core idea with htmx is that you transfer hypertext with controls and structure built in, not just a JSON blob that requires additional context to be useful.
I have just shipped a very useful and interactive app surprisingly quickly for my customer using just htmx with a little Javascript.
All the http verbs. Decent html input controls What else?
If your need really, goes beyond what htmx offers, then you may need Javascript. But in my experience people tend to use the tools they know for their job, not the tools that would be best suited.