I can definitely tell the difference between 1080 and 4k on my 55inch at around the recommended viewing distance. Less so on a 42 inch. A 4k image will look good even on a projector, as you say.

Personally I think 4k (with HDR) is good enough for consumer use, so agree with the parent theoretically just not quantitatively.

It did take around 20 years from DVD to 4k Bluray though.