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
Well, there are recurring costs, such as:
But those costs are pretty fixed.
Hosting an instance, however, is an order of magnitude or two more expensive. Instead of costing up to hundreds per year (not counting dev time), hosting tends to cost hundreds per month for larger instances.
So if people are continually coming to an app, I could see a fixed purchase price being different, and ads are probably enough to support it entirely on a free tier. An instance requires ongoing donations instead.