They could do what other apps have done and release a new SKU for a new business model or with a new feature set that justifies asking for more money. Reeder has done this, for example.

I think JetBrains has one of the most fair and honest subscription schemes where you pay for a subscription but when you stop paying you’re free to keep using the last major version that released while you were subscribed. I think that’s much harder to do on mobile app stores, though.