KWin can scale raster windows of old applications that do not support Wayland. This allows you to comfortably use them on monitors with high DPI. However, the problem is that scaling is applied to absolutely any applications that use X11 instead of Wayland. At the same time, there are applications that do not support Wayland (and most likely will not support it very much soon), but in fact they perfectly support High DPI.

A good example is the Google Chrome browser. It can only work through XWayland, but it correctly handles High DPI configurations. It is even able to dynamically change the zoom ratio when dragging its window onto a display with another DPI under Wayland (at least it works under Gnome)! However, the current version of KWin forcibly scales the Google Chrome window, as a result, the fonts become ugly. And there are a lot of such applications (first of all it is almost all proprietary applications).

In my opinion, the user should be able to include raster scaling only for the selected applications (or vice versa, disable it for the selected applications). Forcing windows to scale only because they work through Xwayland is too crude a choice, because X11 also supports High DPI, although worse than Wayland.