Media Center licenses don't expire though, they're meant to be standalone. Meaning if you have a Media Center 18 license for example, you can keep using Media Center 18 on your computer(s) until it doesn't work anymore. Granted, backwards app compatibility on newer macOS releases will become an issue sooner than later. Same also applies to Linux as well. Windows has the best backwards compatibility, I bet you can still run 20 year old Media Center releases on Windows 10.
So it's not a subscription or an annual fee. It's an optional upgrade to a new Media Center version that doesn't replace your existing Media Center version(s) installed (which you can have multiple versions of Media Center installed at the same time too). But if you don't want to upgrade, you don't have to.