Trey bases his assessment on Apple's published "Top grossing" list. This is potentially problematic as it's not clear over what time period this is based or what other metrics Apple uses to put apps into this list. Apple, like the publishers would like to encourage people to spend more over time than just download free apps so they're incentivized to encourage the implication that in-app is the way to go.

Perhaps Angry Birds is dropping on the pay charts because people are finally bored of the franchise? How many years and different ways will people pay to shoot slightly different pigs with birds?