21.5" is not the norm for 1080p. The 'typical' 1080p display is 24", and the ppi for that is 91.8. Furthermore, 21.5" is pathetically small for most users today to get the full immersive feel in gaming. Its is underwhelming to say the least. Even 24" is becoming small nowadays. We even have many 27" 1080p owners (still have one in another PC) and the PPI for that is 81.6. But its nowhere near a preference to my 1440p displays. I've gone through the whole gamut of display sizes, resolutions since my 15" some 18 years ago.
27" 1440p is what the majority of users here own and the vast majority of them are happy with that size, PPI and performance. Of course a minority will have views otherwise, but they are a minority.
24" on 1080p is standard yes, but that doesn't mean you have to get a 24" one. Instead, getting the smaller 21,5" one will give a better image overall.
So your whole debate on PPI does not apply there.
You mention fully immersive feel. As i explained, that immersive feel of a 27 inch monitor becomes nothing when you are really into whatever it is you are doing.
When i play a game, i get so into it, it's like reading a book, i don't care if the pages are big, small, yellow, white or whatever. All i do is focus on the game itself. Therefor in my experience, that argument holds no ground either.
We can go on about this for hours, but to me, the investment in 1440p is completely and utterly useless. I do not need more workspace, thus it's a bad investment. 4K is where i will be saving towards. To each his own.