A lot of the discussion here presumes that the reason one buys a 4K Tv is to get 4K. But as a practical matter, particularly at the larger set sizes, the manufactures are putting their flagship features in their 4K Tv. So if what you want is their best blacks, best sound, best UI/remote features, you need to buy their 4K set.
I'm in the market for a 79+" TV right now, for example (won't settle for 75"). I have definitely considered the non-4K sets, including the Vizio 80" and Sharp 80-90" models. I have the Sharp in our office and I tried out the Vizio at home, so serious time with both. But the PQ on these sets just doesn't match the best Samsung and Sony sets in that size range even when playing 1080P or for that matter SD content. Those Sony and Samsung's have better black levels, better motion handling, better uniformity, better color accuracy, etc. So even if I cared not at all for 4K, on the basis of picture quality I would need to go to 4K to get the best results at that size range.