Yeah, I’ve got one desk set up with an iMac Pro (27” 5120x2880) alongside a 27” 2560x1440 monitor and the difference between the two is quite visible at normal sitting distance. It’s by far the most visible with a screen full of text, with letterforms being notably less defined and more blobby on the lower DPI monitor, whereas the iMac’s screen looks almost like paper that emits light.

I share your distaste for 16:9 though, I’d love for there to be HiDPI screens at 16:10 (my personal favorite), 3:2, 5:4, etc that allow running your UI at a perfect 2x, but I don’t see that happening for a few more years at least. Hopefully the trend laptops have seen toward 16:10 and other tall aspect ratios spreads to desktop monitors sooner than later.