Yep, this is what we’d like to do with nvultra as well. It’s difficult to do through the App Store, though, I think. You either subscribe, or you pay for a lifetime unlock. And the problem with the unlock is that if the developer ever do wants to do a major version update that they feel should be paid, you have to release it as a whole new app.
I prefer the model that Curio uses where you get updates as long as you subscribe. You own the app, you’re just paying for their (frequent) updates. Which I think is what you’re saying. I need to dig back into understanding how MAS subscriptions work in that regard, as 50% of Marked’s income comes from there, and it’s where nvultra will initially be released. I’ve historically felt that an app like marked didn’t make sense as a subscription, so I’ve never implemented it before.