Using all vector graphics for the GUI so that you can freely change the scale is all well and good, but lots of GUI applications need raster graphics too. With integer scaling, you just have different image assets for the different scales. With fractional scaling, there isn't really a good solution; you could automatically downscale a higher resolution image, but then you get the blurriness problem.
GUIs also use lines a lot. If an application asks for 1pt wide lines, and you're at 1.5x scale, what do you do? Do you draw it at 1px wide, and end up with a line that's too light? Do you draw it at 2px line, and end up with one that's too heavy? Or do you make one line of pixels that's solid and one line of pixels that's at 50% opacity, which gives the right visual weight but makes it look blurry and messes up the positioning slightly? This probably isn't a huge problem, but it's the kind of problem that it makes sense if Apple wants to avoid.
If I zoom to 150% on a website, how wide will a 1px line be?
PDF handles that fine, the web handles it fine, Android handles it fine.
It's only in recent years that suddenly this has become an issue that "can't" be solved.
I honestly don't care if regular desktop apps waste performance, as long as Qt, Android, Electron, games etc are just rendered at the fractional resolution properly.
Really irritating because it makes my Thinkpad X1 Nano unusable under Linux. This is probably why macOS does things the way it does: no room for third party developer error.
Sadly, macOS and Gnome refuse to support it, which is ridiculous, because macOS UI Toolkit is originally based on PDF to allow arbitrary scaling.