OK. I misunderstood the question. I think the idea is about buying a title from Amazon, VUDU, Apple TV, etc. Netflix doesn't do that. They're strictly a subscription service. In that case once it's purchased it can be streamed in perpetuity, or at least while the service is still in business.
I remember when Disney had a digital service where one could buy a title in perpetuity.
A lot of these it depends on what equipment you're using. I have a smart TV where I have access to assorted streaming services, but it has no large storage that can be used to download an entire movie. That's purely streaming. Some equipment with an Amazon Prime app allow the user to do a download of the entire title for offline use, although it may require syncing periodically to the server. I can download a movie from Amazon Prime to the Prime Video iPad app for offline viewing. I've never bought a movie from Amazon Prime though, but I've done the download of "included with" titles, which should work the same. Once it's downloaded if you try viewing the title, it will play from the local copy rather than stream from the server. When I look up the title it will say that it's been downloaded. There's also a way to look for titles that have been downloaded to the device.
However, it depends on the equipment. Some equipment only allows streaming from the server.