Edit: For further explanation see here for a longer description of GTK's release strategy. https://blog.gtk.org/2016/09/01/versioning-and-long-term-sta...
So, I have to use linux.
And I have to use something which supports my 27" 4K monitor at 1.5x scale today, not at some point in the future.
I can’t just not work until this is fixed, I have to deal with what broken tech exists today. Sadly.
Your new API won't be adopted by everyone overnight, that's why it is a dead end.
GTK3 is the exception but you can just let it be blurry; everything else can be sharp.
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...