IMO this is the biggest hurdle moving forward. Even if we somehow managed to get rid of ExtJS in the core manager code, ExtJS is also used in most extras. Extras with a CMP (like e.g. Gallery, ImportX, etc.) use ExtJS, extras for custom TV types (like MIGX or Image+) use ExtJS. And especially critical are extras that change/expand an existing manager page (like e.g. SEO Suite or Collections). It’s hard to imagine a scenario where you could change the core manager code and still have a transition period for tightly intertwined extras to catch up to the new implementation.
What makes you think that HTMX is the way to go? What would you use for UI components?
The current trend is to get rid of jQuery wherever it is still used. So this is definitely not the way to move forward.
For the new version of Minishop (Minishop3), the developers (at least partly) used Vue and PrimeVue (for the UI components) → see the extra VueTools. So maybe it could serve as an implemented example of a possible alternative.