Not everything is a bitmap. Ordinary drawing operations operate on coordinates, so fractional scaling should not lead to any blur (although may miss some pixel-perfect designs).
In other words vectors may be scaled with little precision loss, or they may be scaled naively (render to bitmap and then scale the bitmap).
Yes but the fonts, boxes and lines are all fine. And on the web the image issue is usually resolved by starting hi res to begin with and sampling down not up.
When people complain about desktop apps not scaling they aren't complaining about the odd icon, it's the whole app looking like a smeary mess because it's not using vector based graphics. Fronts, grid lines etc. become blurry.
Also, don't you get the same effect (readable large fonts but alignment changes a bit) when you increase the size of your UI fonts, even without changing overall UI scaling?
Why are people (apparently) so attached to pixel alignments for OS-native GUIs?
In other words vectors may be scaled with little precision loss, or they may be scaled naively (render to bitmap and then scale the bitmap).