The human eye can resolve way, way past 220PPI. Even at distance.
I'm sitting here typing on a surface book with a 13" panel that is 267PPI and I would adore it to be double that. I run at native resolution with no scaling.
For the "average" human eye (20/20 vision) it's something like ~338ppi at 25cm right? If you have better vision (my corrected vision is 20/10 which is near the theoretical max for a human (20/8 I think is the max~?).
It's aggravating that people are like "over 300PPI is the max so screw improving". It's literally just the "max" at being able to discern the spacing between pixels accurately. Higher PPIs/DPIs still lead to an increase in objective clarity and crispness as you are able to improve aliasing etc.
Some of my old devices had PPIs in excess of 520PPI and I absolutely adored it (Note Edge). I would kill for a desktop monitor with similar PPI.
SO yeah, I guess "diminishing returns" is a thing but I wish it was at least a readily available option. I would adore a 27-30" monitor that was ~500ppi.. sighs