This is actually kind of ironic, because Apple's refusal to allow for paid upgrades to pay-once software is actually a major driver of developers switching from pay-once to subscriptions. It's way easier to convince customers to buy a subscription to a piece of software than it is to convince them to buy an _entirely new app_ with a new version number on it, then migrate all their data, re-organize their device so the new version is in the same spot, etc.

Don't get me wrong, I really appreciate the Apple subscription interface, too, but I wish it wasn't the only reasonable way for devs to monetize ongoing work.