Think the fairest model is one where you pay once for the app, updates are free for a set period of time, then you have to pay again to upgrade if you wish to.

Think the fairest model is one where you pay once for the app, updates are free for a set period of time, then you have to pay again to upgrade if you wish to. If not, the old version is yours indefinitely. Well aware that the App Stores do not offer this option, but it is possible if you implement your own billing system on macOS or Windows.