Can you please expand your reasoning about this? Developing major new versions of software costs money, it has never been the case in the entire history of the computer market that one general retail purchase of commercial software would get you all updates of all kinds forever. It's normal that all minor updates are free, and the next major version is a paid upgrade at a marginal difference to full retail. This is economics that both matches people's common sense ("I'm paying for the improvements and another period of minor support, but not what I already paid for") and has favorable economic incentives (upgrades are not guaranteed, the developer does in fact need to convince people it's worth it).

In one of Apple's biggest most stupid and infuriating utter fuckups of all time with the App Store, they eliminated this whole mechanism by offering no upgrade support. But the need to get ongoing revenue for updated versions didn't go away, so devs do what is allowed within Apple's stupid system. They either make it subscription, or they create a "new" app where the major version number is part of the name and sell it again, maybe averaging out the price (or offering a reduced fee for the first week say), or some other suboptimal system (new features get introduced incrementally as IAPs say).

It's not even the slightest remotest bit "scammy" though. To take your own example OmniFocus v3 is a major update following years and years of support of v2, it's completely reasonable it'd be a paid upgrade. And the Mac version, on Omni's store, is a paid upgrade (or a free upgrade if you bought it in the last year, outside of Apple's garbage market you can do stuff like that) for owners of v2 (they can keep using v2 of course). It's just "OmniFocus" regardless though, this is hardly some weird thing. It's just that on the App Store they are forced by Apple to do something else.