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 generally agree, though I could be convinced of recurring payment in the case of high speed APIs that need a lot of updates to keep working. Chasing an API can be a lot of work.
Of course, a solution to that is having an up-front payment and letting people update as they wish–if there’s new value in the new releases presumably they will.
Your suggestion is incompatible with the current infrastructure of either Google Play or Apples’ App Store. It’s nearly impossible to do that you’re asking without severely limiting the app.