Main reason is scaling.
I used to think that 1080p on 24" was about as good as a reference as one could get for desktop use, just scale up or down, but honestly I can see the value in doubling that just for desktop / UI detail, and games of course where they can handle it. Text and UI aliasing is actually pretty obvious these days. May just be me.
Of course that means that 27" at 5k, 32" at... 6k?, and say ~40" at 8k would be alright too.
The big problem with scaling of course, is that it needs great software support from UIs to game engines (and UIs in game engines), and also needs high-bandwidth video transmission support. Which is why I'm not in a hurry to get there myself!
I have a 32" 4k from Acer on my desk; it's not bad, except that without scaling everything is too small, and it's just not high enough resolution for scaling to work really well.
Also, Microsoft, who is doing the best when it comes to display scaling between different display sizes, still hasn't figured out how to make a scaled display the same size as an unscaled display when it comes to desktop spacing for stuff like moving the cursor between monitors.
This is a problem I see with a 1440p 32" monitor right next to the 4k one; while scaling is sharper, I wind up just setting the 4k monitor to 1440p to keep things consistent.