Can't believe how much pissing and whining still goes on over resolution these days. sure in side by side comparisons you can sometimes see a little difference.
But I'd be curious how many people in isolation are really that bothered, or can see the difference. Or perhaps more appropriately, perceive a difference once their eyes and brain adjust. For the record I have excellent vision and I can see a difference, if I'm close enough. But that said I genuinely can't remember how long it's been since I looked at any phone over 300ppi in normal use and thought it was an unbearably low resolution. Quality on the other hand is an entirely different story.
At 1080p on a 5-6 inch device, I consider the resolution more than acceptable. I mean consider how excellent people think 1080p to be on a 60inch screen. It's not so much the resolution as it is how closely you view the device.
This is a, let's say debate, that's long raged on in the AV world. But even then most will ultimately agree that viewing distance is the primary factor. 4K allows you to sit closer to a larger screen than 1080 does. Beyond a certain distance it's nigh on impossible to tell the difference. The human eye, any human eye has only a limited focal distance within which it can resolve individual pixels.
Screen quality is as much an important factor as resolution. It's ability to produce good solid blacks, a wide range of colours, high brightness, good contrast, it's refresh rate and so on and so on.
I generally hold my phone at least half arms length away, so roughy 2 feet, at that distance anything Retina or above looks pretty damn good to me.
When push comes to shove, I wouldn't complain if Apple upped the resolution, if and only if they produced the silicone to match. I don't particularly want my system bogged down just for the sake of more pixels. I wouldn't complain if they went OLED, if they can produce accurate colours across the spectrum, improve lifespan and reduce burn in issues. At the end of the day everything is a compromise, but so long as it looks good, I don't really care. But, hey, that's just me.