No, xrandr isn't sufficient. It still doesn't have scaling information, only size information. Changing the reported physical size of the monitor is a bad idea as it can break other things.
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.
