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
I thought Play was $25/year or something, but it’s been years since I cared enough to check. If so, I guess for Android-only apps, it’s much less of an issue.