xoa, I think you've gotten lost in your own argumentation: besides saying that paid upgrades were not a scam, you were also trying to convince newtacamp that charging money for the OmniFocus v2 to v3 upgrade is perfectly reasonable, if you remember...
You claimed that "developing major new versions costs money" and that "it's normal that all minor updates are free".
So far, so good, but the fact whether v3 is a major new version in anything but name seems to be critical to the soundness of your argument. I have v2 and the v3 trial on my device, and unless changing the icons and moving or adding a few buttons counts nowadays as a "major new UI rework" this app did not in fact go through a major UI rework. Neither does it have any groundbreaking new features, rather it's playing catch-up to Things v3 from more than one year ago.
OF v3 is not worth the 20-30 EUR IMO. I do have Things 3. These guys did do a major UI upgrade when they launched it and still managed to charge one third of the OmniFocus upgrade price for the new app.
AppStore upgrade pricing is not a scam, but it's still complicated to get right and can alienate users. I've given two reasons why in this thread: family sharing for IAPs and features / upgrade price ratio.