I have a different perspective. When considering purchasing an app, I always weigh the possibility the app will go away. This can happen because the back end disappears, or because the app is not updated and Apple phases it out (so you can't install it when you get a new device).

Having a reliable stream of revenue makes it less likely that the app dev will give up on it. I don't love subscriptions, but I do recognize that they help ensure ongoing maintenance/development.