I've always thought that. Pretty hard to justify the extra file weight and cost for something you really can't see. Plus, anyone watching on a cell phone is most likely casually watching.
I'm in the market for a 5K IMac and I'm questioning how much difference I would see in 4K on the 27" screen. Probably see some.
Home theater with a big screen is where the 'WOW' factor is at undeniably.
I'll bet Movie Theater chains are watching all this new 4K stuff really closely.
4K has the potential to really cut into their bottom line.