I've seen a demo of the sony 4k projector at 100", and although it was only upscaled 1080p, it looked very good, since you could hardly spot pixels, even from a feet away.
With 4K content it would be even better (at that size 8K would be pretty much overkill).
Unfortunately I haven't seen at larger screens, but my guess is that it will be very fine at normal sitting distances even up to 150" or more.
So it should pretty much cover all bases.
So If you are to get your movie-collection at 4K, you should be fine, even when 8K arrives. It certainly should be enough pixels as a source for your future displays to upscale up to 8K.
So even if 4K BluRay would be the last physical format, it could live on for a long time.
Streaming at the same quality level is a bit off for most internet subscribers. And with downloads directly from distributors, you won't see the same kind of price-drops as you do on movies on physical discs.
For those thinking that 4K is already overkill. Wait till you see 55" and bigger displays with higher resolution, you will change your mind. Well not for those sitting far from the display, but they will never get a cinema feel at home, as it covers so little of your viewfield. (and there are people stating that even on those sizes at a distance, 4K give a feel of looking thru a window, rather than looking on a display, I guess it has to do with contours and shadows that Full HD does not have enough resolution to paint up, but 4K do, even if Full HD might very well be higher resolution than your retina resolution at that viewing distance).