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, uh, think your point is getting away from you here though, yeah? You can argue about the real intent (and we can pick and choose whatever OS you want), but the fact of the matter is OSes update for legitimate reasons and allowing older apps to run is expensive and/or insecure. App development does not and should never stop. Even Linux is patching vulnerabilities constantly. And new features do occur. Buying an app once is outdated in the connected age.
I understand that keeping backward compatibility forever isn’t worth it. But I think it should be kept for longer than it is now.