"Availability of Purchased Digital Content. Purchased Digital Content will generally continue to be available to you for download or streaming from the Service, as applicable, but may become unavailable due to potential content provider licensing restrictions or for other reasons, and Amazon will not be liable to you if Purchased Digital Content becomes unavailable for further download or streaming."
I think every service that doesn't allow you to download the video in MP4 will have something like this. Amazon just seems to not have exercised this option on me yet.
I can't say the same for the Playstation store. In the future I would love to have a digital collection of DVDs and BluRay backed up to Glacier or something.
https://www.primevideo.com/help?nodeId=202095490#:~:text=Ava....
"Availability of Purchased Digital Content. Purchased Digital Content will generally continue to be available to you for download or streaming from the Service, as applicable, but may become unavailable due to potential content provider licensing restrictions or for other reasons, and Amazon will not be liable to you if Purchased Digital Content becomes unavailable for further download or streaming."