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 think the requirement for constant progress, and the expectation that all software be able to change arbitrarily with a year or so of notice, is in fact a problem with software development.
I do software development all the time, and I find this to be an impediment to my work. I also make the kind of breaking changes that cause this problem.