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.
Yes, I know what it is. Here are some corrections.
1. XCB is a low level binding to the X11 protocol. It doesn't really replace Xlib. Originally that was the intention, but it's non-trivial to take an Xlib program and port it to XCB.
2. XCB doesn't know anything about scaling or even about XRandR, besides the wire protocol. Just because a client uses XCB is no guarantee that it even uses XRandR. It's definitely not a guarantee that it implements scaling.
3. There is no way to use XRandR correctly to allow multiple pitches, because XRandR doesn't have that functionality. All it does is report an estimated size of the monitor.
4. The scaling you're talking about is happening in the client, not in XCB or XRandR, and if it's ever going to work at all it needs to be controlled by some other setting or environment variable. Someone else in the thread posted a big set of those environment variables. Ideally you wouldn't use XRandR at all, there would be another extension.
5. None of the above applies to Qt, because Qt is a toolkit that does (mostly) implement the scaling for you. That's why it was weird you grouped it with XCB.