This study popped up: https://www.techhive.com/article/578376/8k-vs-4k-tvs-most-co...

In summary, with an 88" screen at a distance of 5 feet, some viewers rated 8K content (118 PPD) higher on average than equivalent 4K content (59 PPD), particularly those with better than 20/20 vision (~60 PPD). Most would not notice. It wasn't directly tested but I'd guess most people do notice the difference between 30 and 60 PPD, but only those with above average eyesight will see improvements beyond that.