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
Having settings, viewed posts etc sync between devices requires expenses through a server or service
Unless it’s using built-in cloud storage. Like some password managers “sync” via Google drive or OneDrive, etc.