X11 simply isn't built to do this. If you want this to work, then the last 35 years of clients don't do the right thing and still would need to be changed to use a new extension that behaves more like Wayland does it.
No it actually doesn't, I've heard tons of complaints about X clients not scaling correctly. Sure it might work for the subset of clients that are reading the DPI value the way you intended, but in doing so you've silently broke a lot of other clients.
This comment makes no sense, XCB isn't a toolkit. You might be thinking of something else with a similar acronym.
But anyway, any solution that tells users to only use a small subset of compatible clients is about as disruptive as just switching wholesale to Wayland. It's not the reason anyone is hanging on to the X server.
X11 simply isn't built to do this. If you want this to work, then the last 35 years of clients don't do the right thing and still would need to be changed to use a new extension that behaves more like Wayland does it.