No no no. Nothing about the subscription business model guarantees that an app will be or become sustainable. If I buy an app like this and it works. I can use it until something breaks it. That is true even if I'm the very first and only person who ever purchases it. If I'm the first and only subscriber the app will suddenly stop working a few weeks later when the dev realizes he can't make a living on it.

That stinks for the dev, but it's not my fault, and I shouldn't be punished by revoking my ability to use the app.