In the past I developed a mobile app that was standalone and users had to pay only once. Users then started asking extra features and down voted my app when I

In the past I developed a mobile app that was standalone and users had to pay only once. Users then started asking extra features and down voted my app when I said I didn't want to implement them. When the app lost its popularity and was making barely no money, I realized that I was working for free. My user base paid for a product having a well defined set of features and kept asking for more, for free. Moreover, think about a service that needs to rely on an online backend, for instance to synchronize app data between devices, how are you supposed to do that with a single payment business model?