I love affinity products but I wonder about their plan long term. With the current pay once, get updates for free model they will run out of customers at some point. Adobe sold upgrades back in the day, but that is not a possible path for Affinity, at least not if they mainly sell through the Mac App Store.
Something that you sometimes see is developers offering a bundle that includes v1 and v2 of an app where the price is (v1 cost + "upgrade fee"), which is automatically discounted to just the upgrade fee. This definitely works on iOS, so I figure it would also work on macOS.