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
It’s no win scenario for developers. While you’re ok with “one up front payment and then maybe in app purchases to pay for new features going forward” there’s a whole other slew of voices who are going to complain about being nickled and dimed.
Given those choices (and other factors), as a consumer I prefer the subscription model. If nothing else, it lets me forecast my expenditure and continually re-assess the cost/value proposition of the application in question.