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
But one would like to be able to still play Half Life 2 today, even if Valve weren’t helpfully around to update it. One would like to be able to read an old Word document or display an old blog post along with its scripts. So either you support the old standards and, for active content, the old APIs, or you lose access to anything that doesn’t emit enough cash to pay a person to keep it current.