I haven't actually seen an 8k monitor yet, but a couple weeks ago I was at an Apple store and checked out 5k monitor. PDF text was noticeably sharper and at the same time smoother than on my 4k monitor at home. Both are relatively recent 27" IPS panels made by LG.

So I don't know if I will be able to tell the difference between 5k and 8k in 27" size, but I know that 4k is not enough.

So the first video is that framerate is better than resolution for gaming. Not relevant to this discussion. The second is mostly about how screens and eyes work, I'm not sure why you linked it at all.

8K is useful for crisp lines and text on a big close monitor.

I probably can't see even 320x240, but that doesn't mean I don't like having the extra pixels since I don't stare at one spot the whole time. Upgrading to 4k felt extremely liberating compared with 1080, IMO - productivity is so subjective I'm not sure it can be said scientifically that more screen real estate is necessarily pointless. Plus, imagine a screen the size of a whole wall - if you want to project a life size image of a desktop computer onto it, you can only ever render some small subset of your pixels as its screen, so if you want to render it with a 1080 display you'll need a lot more than 4k. The number of pixels one needs is highly dependent on what one is doing with those pixels.

My favorite monitor is my 42.5" Dell ultrasharp. The screen real estate is amazing, but it has old school 100% scaling. I would absolutely love the same panel dimensions with twice the number of pixels.

That's approaching the visual acuity of humans. I think 8K is going to be what we're going to end up with everything.