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
Microtransactions started with horse armor in oblivion. The fact that people can’t see the clear trend with things like this is directly one of the causes of constant enshitification