This is not true. Linux does not handle HiDPI just fine. Try using 2 displays with 2 different dpis. You get your choice: one screen normal, and one screen teeny tiny.
actually on wayland it does. tough there aren't many wayland distro's.
Good point! I haven't had any Wayland distros work long enough to plug in a 2nd monitor. :)
Linux is not something that handles hiDPI. X handles it, Gnome handles it, Qt handles it. Almost all of them are a bit different, and you have to take care of them separately. There are things that already work together well, e.g. X DPI detection works with Gnome 3, but if you open a Qt GUI (old Skype) then that might very well be tiny. Chrome has not worked well until recently, I don't know if they fixed it already or not.
That's all I needed to hear to know that 2016 (and probably 2017) is still not "the year of the Linux desktop" (coming from a Linux desktop guy until 2004: switched to the Mac because I value my time, I keep monitoring any new developments in my old camp but if I had held my breath I'd be dead.)
GNOME autodetects a high-DPI display and scales well by default, and all modern applications handle this as well. (If you run old non-toolkit applications, they may not scale automatically.) I did find that Firefox doesn't seem to autodetect high-DPI displays, but if you open about:config and set layout.css.devPixelsPerPx to 1.4 (for 1440p) or 2 (for 4k), Firefox works great. (Adjust to taste.)