I'd be annoyed as well. The developer set the price and it was for a perpetual license. Now the developer wants to change the terms. That’s not how things SHOULD work. Imagine that you bought a car….you've paid it off, you're happy with it, but now, the manufacturer has decided that cars can only be leased….and applies the retroactivel. You now have to pay again for a car that you already paid for.While it's of course sad for any user, I'm really hard sympathizing with a comment like this:
So he's been using the app for 6 years after paying 9 bucks and some more for the macOS app. And it's supported him through three degrees. And how he's upset the company who's obviously provided a great product want a stable financial model to help them keep providing a good app. Do they all think these companies can really survive for almost a decade on a $9 purchase and free upgrades after that.
The grandfathering is that they don't have to pay for the first year. I guess you can argue that's not enough, maybe add more time. But still...
There are other options available to the developer. They could have released a new app with the new features leaving the original app intact. They could make new features in-app purchases. They chose a method that will anger its existing customer base.