So, what exactly is it that you expect as "HiDPI" support which is missing? As far as I can recall, it has been possible to configure the actual physical size/DPI for a monitor for many, many years. Any properly functioning X application should then be able to draw things a correct size on the screen, e.g. fonts sized in points or images and figures scaled to actual widths. I think this infrastructure may even pre-date the switch from XFree86 to Xorg. Is your complaint that there are still some applications which ignore this monitor DPI metadata or which do other pixel-based techniques?
Like the previous poster, I stick to 1920x1080 on my 14-inch class Thinkpad. In my office I have dual 28-inch 4K monitors. These have identical dot pitch to my Thinkpad, so each monitor is like having a 2x2 array of my Thinkpad screens. When I made sure the monitors were set with the correct DPI, everything worked exactly as I would expect. Whatever rendered as one pixel on my laptop would also be one pixel on my workstation, and I just had 8 times more real estate on my dual monitor desktop. But, I sit further from the monitors than I do my laptop screen while also using them for much longer stretches of time. So, I adjusted the workstation to pretend it had higher DPI so that things would render a little bigger.
It's been a few years, but I think I may have had to separately adjust Firefox because it has some of its own weird assumptions about fonts and DPI that I assume come from their renderer straddling several different platforms. I also had to adjust emacs and xterm to change from my decades old fixed font preferences to start using scalable fonts.
