Whether or not you see 3D as a good thing to have or not, you have to admit it is something noticeably different. On the other hand, with 4K there is a very high probability most consumers will not see any difference (except to their pocket books). Even if you have 4K content on a 4K set, the viewing distance to screen size ratio for most home consumers is not going to justify the extra pixels.
According to this source: http://s3.carltonbale.com/resolution_chart.html
With 20/20 vision, to barely start to discern any improvement of 2160p over 1080p you must sit closer than 10 feet and to get full benefit of 2160p, you must sit 5 feet or closer. I don’t know too many people sitting that close to their 80 inch sets, and 80 inches is by no means a minimum size for 4K, and smaller sets will of course require even closer viewing distances to discern improvements.
I am not going to call out 4K as a fad. But it is an unneeded improvement for most folks, and I think it is a little sad that many people will buy into it and not reap any benefits.
The best part of 4K displays in my opinion is it opens up the possibility of passive 1080p 3D.
A much bigger deal than more pixels to me is HFR (high frame rate). I know it takes some getting used to, but that is a level of realism that can definitely take movies and television to the next
level of emersion.