In this case I'd actually agree that the author got what they paid for: A lifetime license to Goodnotes version 5. I don't think Goodnotes should have worded it as "lifetime license", the "lifetime" has no real meaning.

Updates should be free, but upgrades don't have to. That's how it worked with software previously. Sometimes you could buy older version of e.g. Office used, and that part we lost with downloads and app stores.

The app store model just sucks for every one. Developers needs to resort to subscriptions, because upgrade pricing isn't supported. Consumers are confused, because why are there multiple versions of the same software?

One issue I do see in this case is that Goodnotes aren't offering a subscription free alternative. That might be due to the AI feature. If that isn't running on device, then that's a recurring cost they placed on themselves.