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 blame Apple for not creating a viable system for paid upgrades; it’s perfectly reasonable for a developer to expect to be paid for a major app update - even if it was largely to support a new OS - but without a subscription, the only way to do that is to launch a brand new version of your app, which loses you all of your carefully cultivated SEO / links / etc. (doing this via IAP is impractical because you can only build your app against one version of iOS at a time; it either supports the new version or it doesn’t)
And I suspect Apple does this because they don’t want people to have to pay money to continue using apps on a new version of iOS, or a new phone; if buying a new iPhone meant forking over $50 to upgrade your favorite apps for it, that might mean fewer people buying new phones.
So don’t blame developers for this, in other words; a lot of them would be perfectly happy to charge users the occasional upgrade fee instead of a recurring subscription, but Apple doesn’t want them to. (they’re also very happy to have their 30% cut of all of that lovely subscription revenue)
Apple and Google app stores had done devs dirty by popularizing the dollar-an-app model. Completely set the wrong expectations about software for users especially younger ones who grew up with this bs.