8K movie releases will be few and far between
If they ever come at all. You'd see the grain on 35mm film at that resolution, so it would only make sense for 70mm. Now that a lot of stuff is shot in 4K digital, you can't magically upgrade it to 8K unless it is almost entirely CGI (and if so, re-rendering an entire movie would be fantastically expensive, and could never be recouped on sales of an 8K version, so it won't happen)
Besides, it is pretty damn hard to tell the difference between native 4K and upscaled 4K on a quality 4K TV unless you are eyestrain close to it. The main improvement we'll see with 4K is HDR, which we could have had with HD but didn't, because reasons.
It is a simple law of diminishing returns - the difference between SD and HD was huge, especially since most also upgraded from analog to digital at the same time. The difference between HD and 4K is a lot smaller, and that's even accounting for the fact that in a lot of cases (at least in the US) it is an upgrade from 720p to 4K which is a 3x jump. The upgrade from 4K to 8K would be a simple 2x jump, on top of the already diminishing returns - I just don't see there being any real demand for it.
Sure, you'll be able to buy 8K TVs, because TV OEMs have to find reasons to get people to buy new TVs when they don't need them. And Netflix will put out a few 8K titles just for the heck of it. There just won't be enough demand for 8K to make it a real standard. Heck, the jury is still out on whether 4K will be more than a handful of major channels, along with movies, rather than the "nearly everything" that the HD upgrade was.