Subscription models only make sense for an app/service that have recurring costs. In the case of Lemmy apps, the instances are the ones with recurring hosting costs, not the apps.
If an app doesn’t have recurring hosting costs, it only makes sense to have one up front payment and then maybe in app purchases to pay for new features going forward
That’s expensive. Increases the attack surface. Degrades performance by requiring more overhead. Bloats the size of the OS. Sure, you can care about backwards compatibility over all of that. But apps will likely continually get developed regardless of backwards compatibility. So there’s still cost.
Again. I’m afraid you lost your point somewhere. Development rarely is ever completed. If it’s truly “completed” then it’s an extremely simple app with no real value and probably not worth anything. If it has value and isn’t simple, then it can always be improved. So hosting isn’t the only reason for ongoing payment. Continued development is extremely legitimate. Is it possible someone might abuse it? Sure. But software development never stops. It will always go into sustainment after release and when sustainment is over, the app is retired.