The Mac App Store (and all of Apple's App Stores) doesn't enable this sort of licensing. It's exactly the sort of thing that drives a lot of developers to independent distribution.
That's why we see so many more subscription-based apps these days, application development is an ongoing process with ongoing costs, so it needs to have ongoing income. But the traditional buy-it-once app pricing doesn't enable that long-term development and support. The app store supports subscriptions though, so now we get way more subscription-based apps.
I really think Siracusa came up with a clever pricing scheme here, given his want to use the app store for distribution.
Which really means up until the dev gets bored, which can be as short as 18 months.
I wouldn't mind something like this versioned to OS. 20$ for the current OS, and ten dollars for every significant update.