Eh, this has been a problem with computer games for about 15 years now. When you buy a game from a digital storefront, they reserve the right to remove availability for it at some point in the future.
It doesn't happen often, even if they remove it from sale then it's still available for people who've purchased it. But there have been a few high profile occurances where games have been changed, or some functionality stops working, or they just plain disappear.
It's the same with Disney+ and prime. Even if it's gone for streamers, its usually still available for people who've bought it, but it's not a guarantee. And of course it's in their T & C's, which you read when you signed up. (What do you mean you didn't read 25 pages of legalese?)
And of course, there's always the chance that the platform will go bust, maybe not likely with Disney or Amazon, but certainly for smaller content providers. At the end of the day, data is ephemeral. Once upon a time you were at risk of your tape player chewing up the cassette, or the DVD getting scratched, now it's the whims of the content gods.