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
Ongoing development IS an recurring cost.
I have zero problem with people trying to get paid for their work, often it is the only feasible way to dedicate enough time to the project.
I’d prefer open source sure, but I’m not all that opposed to small/individual projects not going that route. Especially when it’s not a critical service and there’s an abundance of FOSS choices