I'll consider it enough when I no longer get light headaches reading subpixel-rendered text on a desktop LCD at 18 - 24". My 1080p 22" display has ~100 PPI and that makes my head ache. That 27" 4k resolution display would have 160 PPI, which might be enough for me. But the 1440p 27" display only has 120 PPI and I still get headaches off that.
It is the same thing as FOV in FPS games. Some people get really bad headaches with narrow FOV on widescreens, some don't. If the 1440p 27" display is good enough for you, then just don't buy a more expensive 4k display at that resolution. Or get a 32" 4k display and get more space. I'm gonna get the 27" one because it feels easier on my eyes, probably at a price premium. I'm fine with that, I just want the pixel density.
It is the same thing as FOV in FPS games. Some people get really bad headaches with narrow FOV on widescreens, some don't. If the 1440p 27" display is good enough for you, then just don't buy a more expensive 4k display at that resolution. Or get a 32" 4k display and get more space. I'm gonna get the 27" one because it feels easier on my eyes, probably at a price premium. I'm fine with that, I just want the pixel density.