There's absolutely no reason why Xorg "does not support hidpi". It even supports differing DPIs per screen. It does not support however _overriding_ the DPI ind

There's absolutely no reason why Xorg "does not support hidpi". It even supports differing DPIs per screen. It does not support however _overriding_ the DPI individually per screen, but this would be trivially solved by standarizing toolkits on some other property (either a plain property or a fancy xrandr extension).