It's all relative to your personal eyesight and how far away you typically hold your device. Most people won't notice much of a difference, and yet Apple increased it on the X to @3X super retina or whatever.
I'm a visual designer and am nearsighted, so I notice the pixels on 326 PPI just enough that it annoyed me a little going from a 6 Plus to a 6s a few years ago. I can see the smoother forms of the small typography on things like the time at the top and other fine lines and iconography. I don't notice it much on photos and other complex imagery going from @2X to @3X on my X.
Overall I'm glad they went to @3X, but I see no need to ever go beyond that. I notice the lower resolutoin more on the iPad Pro 10.5", even though I hold it further away than my iPhone, but sometimes I sit fairly close while drawing or touching up dust spots in photos. I also think the retina Macs could use a little more resolution. The problem is keeping things even by scaling to @3X. I don't think the iPad or Mac need to be @3X, but something more in the middle. But that causes problems for designers. Perhaps at some point they just need to make a clean break and redefine their UI scale, but most likely they're just going to end up putting @3X displays in those machines and it will be overkill but 1, the price difference will eventually be negligible and 2, it won't make any meaningful impact on some crazy new future GPU. And again, I bet only 10-20% of people notice it but a good chunk of them are probably visual designers like the people at Apple so it will probably happen, just like super retina did, and Apple can market it as a feature to tick a box against the competition.
Besides, it would be really cool to run a MacBook Pro with @3X retina in @2X mode to have a lot more desktop space for UI elements while retaining sharpness.