Ophthalmologists consider 20/20 a norm on eye exam and will prescribe you glasses only if you dont see 20/20 line, and it's also why all distance calculation charts mention 20/20, because it's a well known norm. Yes, some people and especially young kids can see even better than 20/20 line (like 20/15 and even 20/10) but it's never constat quality. If you would ask kids to look at 20/10 line for long period of time, they will stop seeing it after few minutes. Human body is not a machine my friend, and muscles works differently depending on many factors. However healthy people can see 20/20 line for extended periods of time, because their average eyesight quality will be still above 20/20, so quality fluctuation is not that much visible then. However you cant expect people to see 20/10 for a whole day, it's just impossible.
Now lets talk about real world scenarios
As you can see, some people cant tell the difference even from up close, and not to mentiom from 3 meters when they watch TV from normal viewing distance.
You say there's a marked for 4K displays, because people can see differences. That's of course your opinion, but I think TV manufacturers care about money and their marketing skills just force people into 4K without even thinking about it. These days you cany even buy good quality 1080p tv, so people are forced to buy 4K set anyway. Now TV manufacturers will try to push their 8K marketing to sell 8K sets to people who just already bought 4K sets



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