There is only one sure way to make money as an app developer, iOS or Android: getting paid to develop for customers that will publish those apps on the stores.
And there is only one sure way for those customers to be profitable: give the apps away for free and make money with the service people access with those apps. In the software world think Trello, Slack. Other services, think Uber.
If you want to make money through the store, by selling your app, it's very hard because of all the reasons discussed in the other posts here.
