I feel like the solution isn't paid upgrades but instead monthly/yearly subscription for access in the Mac App Store. The 'appification' of the web is eliminating the idea of major release "versions" of software...it's just an app in consumers' minds and it will always be the same product (with gradual improvement of course). As a result, just charge users for ongoing access to your app and keep them paying by providing regular updates.
I had an idea for a subscription service that would give developers a share of the subscription based on how much their app was used compared to other apps. That may work perfectly for the Mac App Store.
So I pay 10 dollars a month to have access to the MAS and I download 3 apps. I use App A 50% of the time, App B 20% of the time and App C 30% of the time. (To keep the math simple lets pretend Apple doesn't take their share).
App A's developers would get $5 a month, App B's developers would get $2, while App C's gets $3.
Maybe to handle the Pro apps like Final Cut you could have different subscription tiers?
To users, this would be a simple netflix like subscription model. Developers would have an incentive to make their apps more useful and engaging. (I'm sure it could be abused too)
Interesting idea and somewhat akin to how music streaming pays the artists (well, the labels mostly). The challenge, as you allude to, is that software is a lot less homogenous. You only do one thing with a song--listen to it. You use software in a lot of different contexts and it's not obvious that time running is going to give equitable results. And Pro-ishness is also very context-dependent. For me, Final Cut might be a hobby thing. For you, it might be something you depend on for your livelihood.
That said, I do think we're going to see more experimentation with subscription models. Adobe's Creative Cloud is one example. I could imagine some sort of aggregator of games as well.
As with other ideas mentioned, depends on the app. In general, I think people have a limited appetite for subscriptions because every subscription is a "money leak" to be monitored and managed. There is something of a trend in that direction but I'm skeptical that it becomes the standard way to pay for software.
Saas sucks for the user but is great for the developer because people forget to stop the service. Also that protects them from copyright violation, which most users of this site believe is their natural born right.
