Here's an example of UI that's scaled with potrace (a similar algorithm): http://i.imgur.com/jDq4M7e.png (left side is the converted side, right side is nearest-neighbor. Top is 3x, bottom is 2x)
That's with the highest amount of colors/depth that potrace allows. On an i7 processor, it takes a LONG time to compute all of that. Each icon is no longer just a couple hundred pixels, it's a couple hundred vector paths. Even rendering it takes a number of seconds to refresh the screen in inkscape.
OTOH, for icons, you just need to compute that once and then cache the png.
2. We just need vector-based icons already.
In Windows 7 and 8, WPF apps are already supposed to be resolution independent (vector-based).
Old-style GDI+ apps which aren't marked as "resolution aware" (i.e. almost everything) are poorly scaled by the window manager and hideously ugly.
It isn't continuous, but there are more than two scaling levels on OS X:
https://blog.qt.digia.com/wp-content/uploads/2013/04/qtRetin...
More details: https://blog.qt.digia.com/blog/2013/04/25/retina-display-sup...
On Windows the app is exposed to the scale factor (for better or worse): https://blog.qt.digia.com/blog/2009/06/26/improving-support-...
They didn't end up using this approach because the 2x mode is so simple, and will likely be the last scale factor really needed (especially with the additional scaling modes).
The same was true for even longer (IIRC) on the Mac. You could enable it with a hidden preference, but lack of app support, compatibility with existing stuff and lack of hidpi displays at the time made it take the back seat. And vector drawed apps only go that far, you need pixel assets at some point or another.
The 2x scale was just the solution finally used by Apple because of the same kind of issues as you describe for Windows.
But yes, apps are also broken, even very popular ones like Chrome and Steam.
Google almost had support in Chrome 34 after a few false starts, but it looks to have been disabled yet again in current releases.
Now that Windows users are trying to use Retina MacBooks via BootCamp and 2x-resolution Windows 8 tablets like the IdeaPad Yoga2 are out, the DPI scaling issue is finally starting to get mainstream exposure. Most reviews of the IdeaPad called out Chrome specifically and once 4K displays come down in price the gamer crowd will probably jump on the bandwagon as well.
What is needed for this to work is a user setting to override the programs own preference. There is just too much legacy cruft that accidentally marked themselves as being resolution aware even if they're not. Or drop the old flag and create a new one called something like "really really resolution aware"
I haven't played with this stuff in a few years - my housemates and I tried to use a 46" TV in the basement as our web browsing machine for a while and ended up frustrated every time we tried to scale the UI on any platform (OSX, Ubuntu with both Unity and GNOME2, Windows).
You'd think Microsoft of all companies would work in compatibilty fixes for at least the major apps. Like how they worked in so many fixes to make them compatible from 98-XP and such.
TF2, for example, plays beautifully at 3840x2160@60Hz. Scaling up the graphics for an 800x600 visual novel that plays at 1 frame per 30 seconds is computationally possible.
(The good news is that someone has written an emulator for one VN that I'm working through, so I should be able to play with scaling in that way. If I can get it to compile.)