A subscription implies you lose access to the software when you stop paying the subscription.
Buying a license implies you own it and are entitled to use it indefinitely. You might not get any updates but you also aren’t losing access to what you already paid for. Very, very big difference.
While I understand where you're coming from, I think "indefinitely" is a fairly impractical viewpoint in the sense of modern computing, particularly in the context of 1Password. Presumably you'll continue to update your web browser and your OS, which will at some point necessitate updates to the 1Password apps. For example, with Safari 13, which came baked in with macOS 10.15, Apple changed their entire extensions framework and retired the old one. 1Password 6 was built around the old one. So even if you have a license, and could theoretically install 1Password 6, if you're a Safari user it doesn't do you much good. Membership on the other hand would've included 1Password 7, where we implemented a Safari App Extension for Safari 13+ support. Just a counter-point for consideration. Also, for what it's worth, 1Password memberships become read-only when your subscription lapses, but you don't lose access. - Ben, 1Password
So you are a subscriber in reality, it's just your payments a slightly lumpy.