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
Of course they want a constant revenue stream. I want a constant revenue stream as well, isn’t it normal?
Whether the price is worth paying for you is a different matter but wanting to profit off your hard and good work is completely natural to me.