EDIT: The in-between choice would be moving to SaaS (see Adobe), but that's another discussion entirely
In order to do something like that in the App Store, you have to make your next major version listed as a completely separate app ID. Almost no one does this because everyone in the App Store ecosystem expects new version = hit Update All - and the few times I've seen this done, it resulted in nothing but negative App Store reviews.
By not providing anything other than a single purchase price, Apple has not just discouraged the upgrade price model - they've discouraged everything but a one-time flat price or admitting to the customers that they're truly a SaaS model (by forcing the developer to do it as an in-app subscription purchase).
I prefer this simplicity.