One thing that needs to be kept in mind is that viewing distance is a huge factor here.Chart: 1080P Does Matter
says that 84" 4K benefit becomes noticeable at ~9'. Viewing Distance Chart
says ~35'. One is for average vision (20/20!) and the other for eagle vision. Me thinks the eagle vision number is really optimistic.
Where does most the population fall on the vision scale? That's why they call it average
How many people have the option of sitting <10' from their screen and have the space/$ for 84"? Few. (Even if you think their number is low and pick 15' then 84" is still huge.)
We sit ~15' from our 65". Unless we win the lotto we won't be moving nor will we be spending >$5000. By the average chart we would need >100" for 4K to matter (even 1080 is overkill now but you don't see any large 720).
The only way 4K is going to happen in any volume is if it costs about the same as 1080. That's why we have a 1080 when a 720 would do.
Now if I was well healed I could afford a dedicated theater room with a screen 10' from my chair and a 84" 4K. But if I was that healed I wouldn't want to sit 10' from the screen in a small theater so an 84" isn't good enough. Nor is a 100". The only solution is 120"+ and that spells only one thing - projection. Same reason it works in a commercial theater.
If they drive 4K into smaller screens (where people can afford it) then the distance becomes even smaller. A more typical 55-60" needs a viewing distance under 6'!
This isn't the same as the Retina Display from Apple. That makes sense because the normal viewing distance is very close. The Apple 27" display also makes sense at 2560x1440 because the normal viewing distance is 2-3'.
I just don't see 4K being useful in average videophile homes for a very long time. Maybe in 20 years when the TV is a film that you unroll and stick to a wall and is 10'+. So I predict another thud just like 3D.