Let me explain:
From a users perspective, this would suck. It is an antiquated system. "I have to pay for an upgrade just so it works on the new OS?" Think of how many times you've had to do this in the past, and how you felt.
Apple will act in the best interest of Joe User, not Joe Developer.
Instead of paid upgrades, dev's should be providing In-App purchases for new features. Maintenance of the App should be provided for free. Win-win for all.
Now stop bitching about what the MAS should have, and start using the solution Apple has given you! :)