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 wild what lemmy.world has done. If your referrer is lemmy.world itself, a click off their web page, it loads the comment. But if you come from another lemmy instance or just put the link directly into your browser address bar, they reject it with ERR_INVALID_RESPONSE - I can’t recall having seen a website do this to try and prevent attacks.