The smallest readable text is always hard to read, as otherwise you'd go smaller. I'm not sure what that measure proves. Going from 1080p to 2160p makes that same size text much easier to read, because it's much sharper.
You’re comparing a 150 ppi (2 megapixel) 1080p display to a 300 ppi (8 megapixel) 2160p display. Many of us agree that displays in the 100–150 ppi range are noticeably pixelated at typical laptop viewing distance.
But you should be comparing to a ~200–230 ppi (4–5 megapixel) display, which at laptop viewing distances (let’s say 24 inches) is just about at the limits of human perception. Dense enough to not be worth the hit to performance/cost/battery to bother going much denser. The Apple 15 inch “retina” display is 220 ppi, 2880 x 1800 pixels. (That’s the analog of 1620p but at a 16:10 rather than 16:9 aspect ratio.) The Apple 13 inch display is 227 ppi, 2560 x 1600 pixels, the 16:10 analog of 1440p.
I was just continuing with the comparison in taneq's post.
Personally, I noticed the (admittedly very minor) improvement going from ~245 ppi to to ~260 ppi, so I doubt I'd have missed a much larger jump, but what's true for me isn't necessarily true for everyone else.
Up to about 300 ppi (maybe even 400 ppi), it’s still possible for someone with good vision to notice a difference at 24 inch viewing distance. I can tell the difference too. If I bring my face to 8 inches from the display, I can notice up past 600 ppi.
But being able to notice something isn’t the same as benefiting in a substantial way. 300 ppi is well past the point of diminishing returns for a laptop. If there were no competing trade-offs, having a 300 ppi display on desktops, a 400 ppi display on laptops, and a 600 ppi display on phones would be great. I just don’t think they’re worth it in return for a big hit on performance and battery life.
Apple’s choice to take early-2000s ~100–120 ppi resolutions and just double them all (using a 2x mode for legacy software support) got them to IMO just about the current sweet spot for PC displays.
The immediate next things to improve in displays are bit depth (12 bit/channel), dynamic range, frame rate (120 Hz? variable?), maybe color gamut (personally I wish we could get more than 3 primaries to widen the color gamut while avoiding spectrum spikiness, but there’s a big content chicken–egg problem), and power use (I want a laptop with 2 day battery life).
In a decade, more software will be properly designed to handle arbitrary-resolution UI, integrated GPUs will be beefier, display protocols will support higher resolutions and framerates, and it will maybe make sense to hop up to 300+ ppi PC displays.
To be clear, I'm not saying that everyone should use 300 ppi displays, or that everyone would benefit. But you need to remember this goes both ways. Not everybody cares about battery life. Some people want 5 hours, and everything above that is superfluous. Some people basically never even unplug their laptops. Not everybody cares about the performance difference; as long as it renders browsers and office apps, it's fine. Others want 4k because it's a pixel doubled 1080p, and they like gaming at 1080p. Some people never even use the built-in monitor. There is no single, homogeneous sweet spot.
I agree that there are other useful steps. OLED laptops are happening, though scaling slowly. 144hz, variable framerate laptops are happening, and are set to completely take over the gaming laptop segment. Touch screen happened. Power efficiency is improving, a recent step being IGZO. It's not like these other aspects are being ignored.
There's a lower limit determined by pixel size, and a lower limit determined by the angular size of the letters as seen by the user. For me, at least, the two are about the same on a 1080p 15" screen. A little more resolution (1440p maybe) could be useful but even that's past the point of diminishing returns.
