Is 8k UHD or even 4k UHD actually necessary?
It all depends, of course, on what you are watching and where you are watching it. For instance, if you are showing something on a huge screen in a movie theater, you could definitely perceive a difference between 1080p and 4K. If you are looking at your phone, perhaps not as much. For standard computer monitor sizes, it's still fairly easy for many to see a difference at 4K, purely because we sit so damn close to our monitors.

Also keep in mind that there are scenarios in which you need to shoot in 4K in order to distribute in 2K or 1080p. You may need to zoom into one part of your shot and crop most of it out, which you can get away with if the footage you shot was a higher resolution than what you need to output at. This also applies to any footage that you may need to stabilize (i.e. windy drone footage) since stabilization requires cropping your frame. So in these situations, we not only need cameras to shoot in 4K+, but also monitors and devices that can play them back in their full resolutions.

While there are certainly diminishing returns on ever-increasing resolution on small consumer devices, there are people who absolutely perceive the differences between 4K and lower resolutions. Up to 8K and beyond, I'm not so sure, but for the moment, we have not actually crossed that threshold.