If, say, you'd purchased and downloaded the movie on your iPhone 7 and then the movie was removed from Apple's store, could you then copy the movie to your iPhone 8 when you upgraded? If not, then this is about as far from "owning" a movie as you can get.
Remember, if there's cloud-based licensing involved, you're not "buying" anything, you're just leasing.
I don't know about movies, but you can definitely lose apps. I used to own a game called X-Plane that I played on my iPad. Then the authors created a new "version" (as a new paid app.) At some point, I reset my iPad and set it up as a new device and there was no way to download the app anymore as it had been removed from the app store. It is still hosted there somewhere, so if you have the backup, it will download the app from the app store, but it is not "findable" so if you have a new device, so sorry.
Yes, as long as you have a digital copy of the movie you can move/copy it to any device tied to your account. The problem here is he did't have a downloaded copy of the movie.
Remember, if there's cloud-based licensing involved, you're not "buying" anything, you're just leasing.