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 don’t really get how everyone focuses on the ad-free feature as if it’s the only thing that Sync Ultra provides. It also provides text recognition in images, translation in-app, (both requiring constant server work) and will eventually support push notifications (again requiring server work). On top of that, LJ has stated he wants to work on this app full-time, which is only possible if he earns a living from it.
If those features aren’t interesting to a user, there’s always the one-time ad removal option (I’ll admit which is a bit pricey but per OP’s post, is a one time fee and not a subscription).