In 2024, with GNOME 45, Wayland, and 1.25 fractional scaling, regular DPI displays still look better than HiDPI displays. This is a photo of Discord on two laptops side by side.
The blurry one is the HiDPI display from Framework 13. The sharp one is a regular DPI display from Dell XPS 13. Both laptops.
The difference is even more stark in person.
Even the screenshots from the Framework are blurrier than the screen shots from the Dell.
SVG is vector based, you can scale it endlessly. I don’t know how KDE does it, but the only thing I can imagine SVG giving you grief scaling is if the DE is caching bitmaps and scaling the cached versions instead of redrawing the icons.
Caching bitmaps for SVGs is sensible, not updating them when needed is madness. So probably it’s something else.
It is scalable but the icons are still drawn against the virtual pixel grid. If an icon is designed to be perfectly pixel-aligned when rasterized at a certain size, then rasterizing it at 1.25 of that size will cause small distortions if it contains small elements (such as 1 px width lines).