When you purchase a movie from Amazon Instant Video, you’re not buying it, exactly. It’s more like renting indefinitely.

This distinction matters if your notion of “buying” is that you pay for something once and then you get to keep that thing for as long as you want. Increasingly, in the world of digital goods, a purchasing transaction isn’t that simple.

There are two key differences between buying media in a physical format versus a digital one. First, there’s the technical aspect: Maintaining long-term access to a file requires a hard copy of it—that means, for example, downloading a film, not just streaming from a third party’s server. The second distinction is a bit more complicated, and it has to do with how the law has shaped digital rights in the past 15 years. It helps to think about the experience of a person giving up CDs and using iTunes for music purchases instead.

“In the good old days, you purchased a CD, which meant that you owned the medium outright and had an authorized copy that you could do anything you liked with, subject to copyright,” said Dan Hunter, the dean of Swinburne Law School. “For example, you could give it away to a friend, or resell it, or whatever. These days we live in a world where we generally license copyright content, like games and music. This means you’re given a limited right to do things with the content—generally this is limited to playing it on a small number of devices—and you definitely can’t resell the content or even give it away. You haven’t really bought the song, you’ve bought a contract to play the thing for a while.”

So what happens if the company that sold you that song goes out of business? Very quickly, things go from being complicated to becoming a “super complex” problem, Hunter told me. Apple’s document outlining terms and conditions for purchases from the iTunes and App stores is nearly 80 pages long. “Good luck working out if you still have a right to use the music if Apple goes out of business,” Hunter said. “I’d have a hard time working it out, and I’ve been a copyright lawyer specializing in high-tech issues for 25 years.”

For streaming purchases, the unfortunate fate of one’s collection is pretty straightforward: “Let’s imagine Amazon goes out of business,” said Siva Vaidhyanathan, a media studies professor at the University of Virginia. “In the case of streaming videos, yeah, you just lose it. It’s just not stored locally.”

In other words, unless you’ve taken the time to download each title you purchase to stream—Amazon recommends you do this “promptly after your purchase” in its Instant Video Terms of Use, by the way—your access to that film depends on a variety of factors, all of which are outside of your control. Amazon, which didn’t respond to multiple interview requests for this story, doesn’t even have to go out of business for you to lose the film you bought.

In order to keep a film in your collection watchable, there’s a constellation of pieces that must be in place: The software that streams the video has to work, the devices you want to use to run that software have to remain compatible with it, and the film itself has to be accessible on that software. None of these things is guaranteed. The films you buy could already, at any time, automatically disappear from your Instant collection. (Again, that's right there in the Amazon service terms.)

All this signals a larger cultural shift in the way people think about ownership of media in the 21st century, or how they ought to be thinking of it. Increasingly, the purchase of digital works is treated like the purchase of software, which has gone from something you buy on a disc to something downloadable with an Internet connection. “You might think you’re buying Microsoft Office, but according to your user agreement you’re merely leasing it,” Vaidhyanathan said. “You can think of music and video as just another form of software. There is a convergence happening.”

That convergence is built for a streaming world, one that’s driven by an expectation of instant gratification. “One of the things we’re doing increasingly is opting for convenience over dependability. And we’re doing it somewhat thoughtlessly,” Vaidhyanathan told me. “We have to recognize that it is temporary. Anything that is centrally collected in a server somewhere on Earth is ephemeral. Even if Amazon doesn’t go out of business in 20 years, Amazon will not exist as we know it in 100 years.”

Even in the realm of personal collections, the changing media landscape has implications beyond movies and music, too. “If Amazon were to go out of business, all of my Kindle books would be inaccessible [because] Kindle books are tethered to Kindle software,” Vaidhyanathan said. He suspects that if Amazon were to go under, hackers would quickly figure out how to unlock the Kindle format, and hopefully salvage materials as PDFs or plain text that way. “It might end up completely liberating that content, which of course, would drive publishers crazy,” he said.

The key, according to media scholars and lawyers who focus on digital rights, is to remember that even today’s most influential tech giants will eventually fall. That’s a prospect that may be easy to acknowledge, but is strange to imagine. “We depend on Amazon so much at this point that a world without Amazon would be kind of terrifying,” Vaidhyanathan said. “Amazon has done so much to bully both readers and publishers. And yet if it were to collapse, it would cause chaos.”

At the root of that chaos would be the immense loss of media, and the wholesale disappearance of works—not just from personal collections, but altogether. “At the start of the 22nd century, we are going to find ourselves in a situation with huge gaps in knowledge and culture. Because none of these companies will be around.”