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
If I understand correctly, every sync feature that requires the subscription (and cannot be purchased by a one time fee) requires the sync dev to run a constantly online server. Translation makes calls to translation services that cost money, push notifications require a push server since Lemmy servers don’t include support for it, etc. Removing ads doesn’t cost sync ongoing cash which is why you can get it for a one time fee.
Seems reasonable to me.