What makes you so sure about that? Until recently I used to work on mobile games with in-app-purchases. There are a small number of users who pay absurd amounts

What makes you so sure about that? Until recently I used to work on mobile games with in-app-purchases. There are a small number of users who pay absurd amounts for virtual goods. Making your game free increases your downloads such that even if only 1-5% users pay anything you could be making a lot.