[phrom]

Look at what Jetbrains does. When you complete a 1 year subscription, you get a permanent license to a version of the software you're subscribed to, that is 1 year older than the latest.

It is at the same time a subscription to the latest updates, and a one time purchase of an older version that will still be able to open your files, if you drop the subscription. Or the company disappears.

I think that is the best option for a subscription. If you go the one off purchase route though, consider also adding a "grace period" where users get free upgrades to the next version or at least a good discount. For example, if I buy your $50 dollars software, and the next week you show up with a new version with really cool features for the same $50 dollars, I would feel scammed, especially if you hadn't made any announcements that the new version was about to come out. Try to either have a release schedule where you announce a month or two before they're out, or to offer a month or two or free upgrades if you someone buys a version right before a new one comes out.