Previously the user paid 1.99$ for unlimited use, forever.
Now you nickle and dime users, forcing them to think about credits, what the value of the service really is, etc.
What's to stop an unscrupulous developer selling an app for, say, $1.99 with the (implied) promise that the app will work forever and then, once people have bought it, turning off the backend so it no longer works?
Is this illegal? Would Apple shut down the developer's account?
Many companies do this with games that have a strong online component.
I suppose a relevant study would be Microsoft Zune/music store, and/or Yahoo(?) Music that shut down and killed the DRM servers, effectively removing music people had "bought"?
An earlier Microsoft DRM that was used by a bunch of services (PlaysForSure) was indeed shut down, but it's not the same thing as Zune.
However, there are some apps that do have a model of $xx for unlimited use, even where there are ongoing costs.
This is sustainable if the app continues to gain new users, hopefully at an accelerating rate. It works like a pyramid scheme where the new users coming onboard subsidise the cost of the existing users.
The other model might be $1.99 for unlimited use, and then release V2 of the app with new UI/features, and do an annual release model. It works for many apps, eg developer tools like IntelliJ. Would be interested to know if it works on app stores without getting grumpy user feedback though.
For me, it's a bit like mobile phone data charges. When I used to pay per mb, I hesitated to use cellular data at all, but now I have a (high) monthly allowance, I now use my phone a lot more, even though the cost is higher.
Perhaps the developer should try switching to a pay-monthly kind of subscription, at least then the users aren't having to make that 'is it worth it?' decision each time they want to use the app.