Most people are fine with streaming even if it is only in 720p or something. Pretty much every TV is saw last week in an electronics store was already 4k so that it would make sense looking at the TVs people get in 2016 but I'm not sure if most even have an idea what this is about nor would they start to buy 4k Blu-Rays just because the TV they bought has 4k.
The reason for why 4k Blu-Ray player are so expensive is probably that not many are interested in it. If it would be a product people would buy in masses because they like it so much, the price would be already much lower.
But not sure why you said "How is no one interested in this?" because the thread I read about the new PS4 was full of people commenting with "day one purchase" and stuff like that. You just can't expect that as many will say "I get this" as if a completely new console is announced. It's after all "only" a console which may play Blu-Ray in a higher resolution and games which come also on the PS4 which people already own just with better graphics or more fps.
It will sell, just not as good as if it would be a PS5 and that should be obvious.
