I think there's a middle ground to be had though. For example, if the average user sends 1 article per day, offer 400 articles for $1.99, and in theory you have a recurring billing model that also doesn't require too much worrying from the user's point of view.

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.

This is a really good point.

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.