Yep, the downside is the m1 iMac is double the price of the intel i7 21.5 4K. How do you got used to the lower ppi? For me the difference its too clear. I had a 27" 4K, and after I see an iMac Screen, immediately I sold my monitor.
Well, basically you don´t have to put them side by side.
4K at 2560 is not perfect, but is good enough and better than QHD.
A good 27 5K display from apple is a first day purchase for me (if it happens).