I think the way things will play out, the Ultra High Def Blu Rays will be worthy of collecting for these reasons. Just my theory on how it will go.
Its not so much about 4k..more resolution...because if you sit 8-10 feet away most people won't notice the higher resolution. Sitting close to a big tv or a computer monitor, you'll notice more.
BUT what the Ultra HD formats are going to bring us is High Dynamic Range and Wide Color Gamut.....which should be blow away great for Movies especially.
Sure Netflix will try to stream that, but they still are going to have these issues......compression of the stream to save bandwidth, and the fact that monthly data caps are part of the deal with Comcast etc. A "real" UHD stream would suck bandwidth like crazy, but a compressed version might not look at good as UHD Blu Ray to the point where people might want to own it/rent it.
Of course, MP3s have shown us most people will take volume and convenience over better quality sound. So it still will be limited to people who are really into movies and collectors.
I personally prefer the BluRay picture, and I do notice the difference but I don't rewatch a lot of movies or shows....so I don't really want to sink the money into it. My Blu Ray player died and I just don't rent anough to buy a new one, even though it looks great.
With 4k though, if they ramp up the content on UHD Blu Ray fast with the HDR and WCG......it will look spectacular and might push it ahead.