From a TV/projector point of view, 4K is nigh on 10 years old now. Early 4K TVs may well have been 4K in resolution, but they soon found themselves incompatible with the streaming services later on when they started to stream 4K, as the parameters and tech had developed and changed over the years. While 4K has now settled down, it is still not a widespread standard. 8K really is pointless at the moment. It may not have the teething issues that 4K initially had, mainly due to the fact that HEVC coding used on UHD Bluray can handle 8K, although I’m guessing the streaming services will have to upgrade the coding they use.
Think of 8K this way, whatever size 4K screen you currently have, you could have an 8K one that’s twice the width and twice the height, and with 8K material the picture will look just as good.
I can see the difference between 1080p and 4K on a 55” screen, so I have no doubt that the difference between 4K and 8K could be seen on something like an 85” screen, but really, I would say 8K is more suited to home projection, probably using screens upward of 100”. And for those already watching 4K on a 100” projection screen, you could have a screen twice the width using 8K and have the same quality - now that really would be something.