The average person would already have to be closer than 10' from a 150" screen to get the full 4k resolution effect. I do not think many will be sitting 3 feet away.
They will not be necessarily sitting 3 feet away, but they will be walking 3 feet away in store show rooms or at Super Bowl parties and the 8K sets will have that WOW effect!
And I don't buy the "average person" argument in general. Most people can't tell the difference between 720p and 1080p image. But go find 720p sets now.
Most people can't hear sound frequencies above 12 kHz or even 10 kHz. But we now have recording with sampling rates of 96 or even 192 kHz.
If you can make an argument that 8K doesn't make sense at 120" then you should be able to make a similar argument that 4K doesn't make sense at 50-60".
But guess what was the original subject of this thread? 50 and 58" 4k sets!
I think 8K will be to 4K, just like 1080p was to 720p. It will eventually replace 4K, whether the average person can tell the difference or not.
But no need to worry. We are still years away from that!