I think a lot of steam results are from laptops. 1080p still seems very popular on the cheaper laptops out there and for awhile 1080p was the way to get 120-240+ hz monitors but that is now changing . I also feel that monitors are now finally dropping in price for higher resolution/ size monitors. I bought a no name 32 inch 4k monitor like 6 years ago for $400 . Now I can get a 42 inch LG 4k monitor with free sync for $200. Of course I saw some $50 1080p black friday monitors in the 24 inch range but i'd imagine that is the minority of purchases now.I'd be surprised if that's true for monitors. It's definitely true for tvs. Ultrawides are making a mark, but they're kind of replacing dual monitor.
Steam hardware survey is 60% 1080p (those will be 24") and 15% 1440p (those will be 27"). 4k displays are like 3% (likely 32" or more). The top selling gaming monitors are all going to be $250 or less.