The large majority of the current movie catalog was shot at what amounts to 1080p resolutions, and will derive little benefit from upscaling. Most run-of-the-mill cinema distribution was 2k, and much of it still is, I wager.
So I wouldn't bother trading blu-rays in for 4k resolution media. In the large majority of cases, the master media was 2k to begin with. Why pay for upscaling when you'll get it in the player anyway?
I fully expect 4k to take over, since recent feature film material was shot in 4k and higher, and there will be 4k blurays, but I predict that it will go the way of SACD and DVD audio for a long time. It win't fully take over for TV's until its cost is close enough to 1080p's to simply retire the latter by force. (No more screendoor effect.)
New material will be shot in 8k and will look awesome - but what budgets but the largest will support it?
And then there's streaming. Only now is there enough bandwidth available in most markets for current 1080p delivery to finally approach full utilization of that resolution instead of wasting it in compression artifact degradation.
The classic cinema format - literally, moving pictures in a frame of what amounts to a window - is close to maxed out at 1080p in much the same way that stereo audio is maxed out at 44.1kHz, 16 bit. Yes, you can do better, but the difference is only relevant to an ever-shrinking niche population.
4k is the last stop for final consumer delivery of cinema style content. After that, further improvements will likely be driven by dynamic range (I'm already seeing this), greater bit depths, and possibly going to six primary colors. Then the picture-on-a-wall will be perfect, and retro looks, where filmmakers digitally reintroduce all the flaws the electronics companies sought to eliminate, will be all the rage.
Where I expect the real demand to come from for higher resolutions is computer displays (for both desktop real estate and for high-PPI displays), and for VR applications like the Oculus Rift. That could be big; I'm attending an industry panel on it tomorrow night in Los Angeles.