if you had something like a count that was outside that area you could use an event, an out of band swap or just expand the target. we also have morph-based swaps that make it possible to retain more UI state while expanding the target area, if you'd like
if you have some sort of visualization, you'd recompute it server side and stream it back, or use events to trigger an update
adam james has done some cool stuff w/clojure and htmx visualizations:
https://twitter.com/RustyVermeer/status/1701048955105391044
a little bit of hypermedia-friendly scripting can go a long way:
https://htmx.org/essays/hypermedia-friendly-scripting/
you are correct that htmx isn't for every application, but it can likely pull off a lot more than you think, and at much lower complexity levels than reactive options, which has been borne out by real world experience:
https://htmx.org/essays/a-real-world-react-to-htmx-port/
as far as htmx being future-proof, I would point to it's predecessor, intercooler.js, which was released in 2013 and is still actively supported, simply because the code isn't hard to deal with. you could probably figure out and maintain htmx.js in a day or two, as a good JavaScript developer, something that can't be said for most other javascript frameworks.