Not an Apple App Store vendor, so my information is sketchy at best, but it sounds to me like in-app purchases are only a little more complicated than listing something on the app store in the first place. If the money is the same, and the dopamine hit for the latter is greater, of course everyone is going to 'race to the bottom' with in-app purchases.

Apple has not provided any incentive for people to retain customers by any other mechanism than in-app purchases. Not on iOS, and now not on OS X. Strictly speaking, Steam also doesn't have a way to give a discount to people for upgrading, and yet I've bought several sequels at a modest discount because they support bundling and the bundles are prorated.

However, while a bundle of Red Dead Redemption 1 & 2 might not need an explanation, a bundle of Photoshop N and N+1 is only good for giving a discount for upgrades.

If Apple is interested in turning back this Doom Clock, then support for bundling, and taking a smaller cut of app purchases than in-app purchases would, I think, function as a carrot and stick combination.