I'm on Windows and can confirm 1. is an issue. Windows lets users scale UIs to be readable at high DPI on small screens. Doesn't work on all UIs (e.g. QGIS). So maybe not all OS's, but two important ones.
> You could say the same for any arbitrary DPI; 96dpi isn't "Required", we got by fine with 72dpi. It's all about ergonomics as far as I'm concerned.
I think the point the parent is making is that human vision has limited resolution. I.e. for a given screen size & distance from the screen, you cannot notice any difference in DPI past a point. The parent is suggesting that 1080p & 27" with a typical viewing distance is already higher resolution than the eye can resolve. Looking at my 1080p 27" screen from a metre away with 20/20 vision I am inclined to agree!