I think the way Jetbrains does it is ideal - every chunk of 12 months you pay for with your subscription gives you permanent use of that version.
It's the only solution so far that doesn't lock customers into a subscription that may stop giving them value commensurate with what they're paying, while giving developers a recurring source of revenue that incentivizes continuous improvement.
I've seen it work wonders for Sketch, which seems to have increased its pace of development after switching to an update-subscription model (as a side note, the lack of a clear, standard name for this model may be its greatest disadvantage). I use an active subscription at work, while I have a slightly older version that I've currently let lapse on my personal machine because I only need it occasionally. Between subscriptions and major-version upgrades, it really is the best of both worlds.
The maddening thing with Apple is that this model would slot neatly into the App Store. All they'd need is a new type of subscription that gates App Store updates. Apple already keeps old packages on the server, as one discovers if they download a purchased app on a device stuck on an old version of iOS – the only thing missing is the will to implement this model.
In any case, subscriptions aren't directly equivalent to upgrade pricing even when an upgrade is provided every subscription period because in an upgrade pricing model the end user can always choose to stop paying and continue to use software they already bought.
I think that ignores how little most developers make in the app store. They aren't rolling in money, most are just trying to figure out a way to eke out a living.