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
There’s one thing you’re forgetting; most applications aren’t just code-and-forget anymore. Updates need to be made to fix security issues, adapt to newer versions of the API, etc.