For some reason I misread your “tiny ui with a huge viewport” as a tiny view, and thought you meant it would have been surface cropped.
That said, win actually does have support for larger UI rendering (albeit limited to a couple settings only, 125% and 150% I think), and it works fairly well on most apps, but not every app out there is written to support such things. While the API could be better, and there could be more options, this is no different from what Apple offers (with more options).
In the case of something like game engines and apps with their own widgets, that simply doesn’t apply, as they either don’t use the standard widgets, or use appropriate dx calls provided for app scaling already.
Win8, which has to cope with more formats with the whole tablet thing, does even better than win7, but again, not every app takes advantage of it.
I have my uhwga monitor at 125% for the record, and it renders every app I use just fine, except those that use their own widgets instead of something like a recent Qt (IE: Softimage’s menus).
Some icons (which again is on the dev side of things) scale ugly because there is no large enough or multisize one provided, but I can’t say it matters much.
You’re laying at MS’ feet the whole issue, when it’s actually 80% on the app developers’ side. Win has provided facilities to deal with this for years now, and their own products using the same framework show it works well (as do other products). Again, could be better, but saying it’s missing entirely is wrong.