There are plenty of ways to maintain state, including server store, sessions, localstorage, cookies, etc. Say you want the user to be able to customize the app layout: that doesn't need to be in the URL. Now say you provide a search functionality where the user can share the results: now your search criterias definitely should be in the URL.
It's not a black or white, one actually has to think about what the application must achieve.
> modern UI where you might have many different zones, widgets, popups, etc.
This is completely independent from the HTMX matter, but not all your application functionality has to fit one screen / one URL. There's a thin line between "modern" and bloat. Unfortunately this line is crossed every day.
> React / Vue that all provide their version of a state store that can hold the state
And many times they duplicate what's already available server-side.