The problem with downloading media is likely that it is a proprietary format such that only that platform/app can access it. As you iterated other problems include the ability of the app/program to "check" the validity of the media and the account and either stop the program from accessing the downloaded content you have or deleting it entirely. A problem that happened to me is I downloaded movies in one app and when I tried to cast it to my TV..... wouldn't work at all but in another app it did. If they were normal data files able to be read by any program that can read "normal" media files that wouldn't be a problem at all.Not an apples to apples comparison, but reminds me of the Photobucket fiasco recently. All the photos shared for "free" for years suddenly went away overnight, turning so many forums into wastelands of garbage.
Even music on Amazon Prime. I have a decent amount downloaded, but sometimes you go to listen to something and it's now gone.
I guess what I'm saying is in the end they have the ability to control what you can watch/view/listen to that is supposed to be "yours" but you are only able to "rent" it for the rest of your life at their whim. It is quite possible with some of this "correctness" going around that some titles you have could "vanish" because they deem it "hateful" or whatever.