Continuously evolving? Strong disagree.
Your users expect a reliable product that maintains compatibility with the latest OS.
The app’s appeal is its simplicity.
When there is a major OS change, charge for a compatible upgrade. User who upgrade their OS will gladly pay to continue using the app; such pricing is common and understood.
Given that, there should not be a need for spending resources on “continuously evolution.” You got the app right.
The idea of a subscription is contrary to the ethos not only of Ulysses as an app but also writing as art. You invest in a good tool, like a notebook or a typewriter, and you expect to use that tool for a long while.
To put it plainly: saying your users expect continuous evolution is patently incorrect.
This is not a move driven by sustainability. Rather, it’s driven by a desire to show rapid revenue increase.
tl;dr It’s a writing app. You don’t need to use agile development.