I was just trying to get clarification on Affinity's policy as this information was difficult to find, but yes, there are plenty of examples of App Store apps that are yours to own after purchase, for all forseeable updates. Even off the App Store - FL Studio for example is over 20 years old and has always offered lifetime free updates.
As far as whether this is a good business model or not, I believe this would be up for debate or at least analysis. Free lifetime updates could motivate sales, especially when users may be tiring of the subscription model of the main competitor. It could also encourage more users to be part of an ecosystem which could then boost sales in add-ons/extra packs etc.