Again, it is not a matter of new API! New API solves exactly nothing. It won't solve the libXaw/Motif/GTK2/GTK3/whatever apps. They still have to run correctly.
Your new API won't be adopted by everyone overnight, that's why it is a dead end.
Those old toolkits can just be be deprecated (most already are). Most of them don't even run on Wayland: by rendering through XWayland, they will be blurry anyway. You can just continue to have a fallback path that just makes a blurry mess.
GTK3 is the exception but you can just let it be blurry; everything else can be sharp.
Sure, they are deprecated, but Xwayland has to display them at correct scale, still.
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...
Your new API won't be adopted by everyone overnight, that's why it is a dead end.