Yeah it's an interesting dilemma, and one that I didn't encounter until attempting to upscale a WW2 game for to play on a WQXHD screen (1600p). Standard QHD still works reasonably well, provided the map images themselves are large enough. It works alright at 1440 even with no upscaling of the font from 100% to 125%, cause the UI is still large enough there, and the actual font sizes in game can be increased within tripleA. But it gets harder as you go up in res.
This is one my favorite graphics on wikipedia
https://en.wikipedia.org/wiki/4K_resolution#/media/File:Vector_Video_Standards8.svg
The hard and fast rule is that the new standard will give 4x the size and 2x the resolution, over whatever previous standard. The previous standard in this case would be FHD at 1080p which most peeps still use for gaming, like especially on laptops. Also there are features like DLSS and whatnot to smooth over this transition in gaming, though obviously tripleA isn't the sort of game we're talking about there really.
Also, the issue is going to be less pronounced if one were say running tripleA through a giant TV or a projector which will soften the visual, as opposed to say a laptop or gaming monitor where the user is right up on top of the screen.
The issue is not for all images in tripleA, just the graphics inside the UI itself. So for example the map visualization (or anything displaying within the map field of view) this stuff all upscales normally. Using a higher resolution image for the map, you will see that scrolling around the map will be very clean. Just like watching a TV show at 4K or at a higher framerate, you'll see less motion blur when dragging around the map in the UI. This is actually pretty cool, and will instantly make the player feel the upgrade, but then the UI graphics works against that feel (map will look ultra clean, but UI will have that jank.)
Some of our larger maps (larger in terms of pixel height/width) should be fine. For example the UHD global map even at like 5K ultra wide or 5K UHD will still work, because the map image is tall enough that the player will still be able to zoom out to a 50% mapview at least. The text in the UI will also work fine at that scale, because of the way font scaling works (basically as vector for fonts) but the tiny images in the UI, say units or icons upscaling as raster, those will hit the ceiling very quickly when doubling like that.
Also maps which are not tall enough to allow any pulling out in mapview will become sort of unusable without an upscale to the map dimensions. So for example one of our smallest maps is v3, it was drawn back when many monitors where still at aspect 4:3, or when HD 720p and widescreen displays were still relatively new. If you imagine playing a map where the zoom is locked at max zoom-out, with no scrolling/dragging (no zoom in/out from there) you can get a feel for the issue. For the actual maps, we can deal with this by upscaling the map image (basetiles/relief) outside of tripleA to produce a larger/cleaner image, but for stuff like unit icons we're much more limited. 54px is a hard ceiling right now, we can provide a larger upscaled graphic with interpolation, but the game will simply crop it inside the UI.
To me the minimum for anything UHD inside tripleA is at least some give for the map zoom. It needs to have somewhere to go from 100% mapview there (e.g. the map image needs to be larger than the intended screen's display). If not, then the only real option there is to downscale the display resolution of your screen. We have a little play here, because tripleA 2.6 can zoom to 200% mapview now, but there's still an issue anytime the screen and the map image approach 1:1, since your zoom out will just be the map at scale taking up the entire screen at once. Ideally you'd want the map to be at least twice as tall as the intended display, so that the drag and zoom stuff feels normal, but this is limited by the UI upscaling. In short a UHD map would still be serviceable at much higher resolutions say 5K or 8K possibly (even if the map scaling range isn't nearly as broad) but the UI is going to get stuck (for anything other than Font), and start bumping heads on that ceiling.