Imagine a mosaic. The more details in this mosaic and the smaller they are, the more the mosaic looks like a photo or reality. Here's the same thing with increased pixel density - it's just smaller cells that allow finer details to be shown. But if technologies have no limits (except for physical ones), then our bodies do. Progress has reached the point that when you just look with your eyes, you see even less detail than now on all these superdense matrices and so on. Therefore, it does not matter for the eye whether there will be some kind of increased density or not, it still simply will not see some of these pixels. For us, it will simply be a smoother picture with a higher sharpness than on the adjacent monitor. But why then the eyes get tired? - Incorrect viewing conditions - for example, you have very high monitor brightness and very low lighting in the room. In short, imagine that you are shining a flashlight in your eyes - here is the same thing. By the way, the same applies to lamps and windows - make sure that they do not fall into your field of vision when watching TV. Too high saturation, contrast - such colors are not typical for the eye, so he will try to correct them to normal. (Remember those awful neon T-shirts? Now, imagine that you are in a world where only they are around you ... O_o) Actually, while the eye tries to correct this oversaturated and oversaturated color, it gets tired. In addition, one of the reasons may be 3D, especially if it is pseudo3D. That is, when it is just a TV function, and not filming from 2 cameras. But this is a separate topic for conversation, if you need more details - write in the comments, I'll tell you)
Some nonsense. After all, in life it is not more difficult for you to focus, and there is no difference between what you see and, for example, a display with 500ppi at a distance of 50cm from your eyes. But from my own experience, I can say that when reading from a 24-inch Full HD monitor, the eyes sometimes stop to examine the unevenness of the font.
In general, if the eyes get tired faster, it is unlikely that you cannot distinguish between pixels. Maybe from increased contrast or brightness, but definitely not from density.