One of the issues in X11 is, that the display server can tell the clients what the screen dpi is, but it never knows, whether they honor it -- and they never tell. Many of them don't, some do, so you cannot have uniform handling for them, and if you find a mechanism where they can tell, how do you retrofit it back? You won't.

So yes, putting the mechanism into current toolkit is easy, but that means that the linux desktop is going to be broken for decades to come, because the old client won't disappear overnight. If they did, we would be in pure Wayland for years already...