pp Store and the IOS App Store do not allow paid upgrades, correct. However, that is why you end up with "1Password 6" as one product in the app store and "1Password 7" as another product.
Is that "upgrade pricing?" Of course not. Everyone pays the same price to buy the next version, if they had never bought a previous version, bought the very first version and every version thereafter, of just bough the previous version 2 days ago. Subscription pricing doesn't fix the lack of "loyalty bonus" upgrade problems. It only addresses the "guy who bought the previous version 2 days ago" upgrade problem (which, honestly, is solved by said individual asking Apple for a refund, or by the dev issuing a warning on the app description prior to the next version coming out or drastically lowering the price of said previous-version with the warning about an impending upgrade coming soon).
Subscriptions keep people from deciding that they don't need this update because there aren't enough features. Instead, they pay no matter what features are produced, until they don't want the product at all any more or decide to move to a less costly alternative.
If you care about customers and want to put your own skin in the game, you would go the Agenda model: $20 buys you all current features plus new features for the next year; after that you keep those features but don't get any more new features (but do get new versions, just with the newer features disabled, so you also aren't running 2-year-old insecure software) unless/until you re-up for another $20/year of upgrades. Works great, and puts skin in the game on the side of the company, which can't hold my data hostage for their subscription but instead needs to earn it with compelling new features.