Streaming IS probably going to kill physical media, but it will be many years before we are streaming at the quality of a current UHD disc, probably after the next level of compression is invented and source media is 8k, just as current streams of 4k uhd are just about as good of quality as 1080p off a spinning disc, with the only edge going to a wider color gamut. Even if various companies could deliver the bandwidth, they still aren't going to kick out an h.265 at 35-50mbps, as well as full Atmos/truehd audio. And if they did, even my fiber optic gigabit internet pipe would fail to consistently deliver that stream. Sadly, it seems like we keep losing quality as we move "forward".. And it doesn't matter to the next generation because they all watch the content on their 6" screens a few inches from their eyeballs.
By the time we have a regular flow of "8k" media, it will ALL be delivered via a compressed stream, and while everyone will think it is so great (on their 65" tv's or tablets and phones a few inches from their faces), the fact is it will be worse overall than a UHD disc. And only those of us fortunate enough to have truly big screens in our homes with projectors that can play the content will be left out in the cold, hanging on to what UHD media we managed to collect while it was still abundant...
Of course, in the shorter term, UHD's will continue to grow, and as we have seen in the last year, a significant number of old titles will get released in UHD, some even actually properly upscaled and improved.
As for cost of a 4k UHD disc, it is usually 2-3 dollars more than the blu ray, maybe 5 at most, and when you think about the cost of going to a movie, it is a good deal. Take your wife and kids to a premium venue, first release movie, and you will be dropping $100. $20-25 is cheap in comparison. Heck, even a date night at a public theater will cost you at least as much as a UHD disc. What makes it expensive is when you compare it to Netflix.
And the only difference I see at WalMart is the bluray "bargain bin" is now $8. They still have just as many blurays and 4k UHD's as they did 2 years ago, if not more.
Here's the line that will make me sound like a pretentious ass: The fact is, if you think streamed 4k UHD is "as good" as a UHD disc, you don't have a good enough theater to see the difference, and should probably stick to streaming. Sorry, I had to say it.