You don’t own digital media for the most part. When you ‘buy’ a movie digitally you’re simply purchasing a license to stream that movie at home.
Sony can’t go into your home and take your DVD. Sony can sign a contract with the IP holders of Mythbusters that allow them to sell licenses for it. The contract that Sony and the IP holder make can stipulate that they can’t stream it to anyone who had previously paid for it after X amount of years if they don’t renew their contract.
When you buy that Mythbusters license from Sony in the TOS you’ll likely find that there’s likely text reflecting this reality.
TLDR: You’re not buying the movie you’re buying a license to stream that movie from Sony so long as they have a deal with the IP holder.