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
The difference is that software nowadays is interconnected. Sync doesn’t exist on its own, nor does Lemmy. And if one of these links changes, chances are, that something else needs to change to keep up. You’re talking about standalone software that that exists entirely on its own. But that’s not what this post is about.