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
All apps have a recurring cost if the dev is continuing to develop the app. At the scale these apps are working the labor the dev puts in in the most expensive part. Plus Lemmy is continually updating so to keep the app working the dev will need to continually update.