Mobile app revenue. Ads where Apple doesn't get a cut vs IAP where Apple gets 30%.
Ads and IAP aren’t the same thing. The money doesn’t even come from the same places.
The battle is over attention. Users have a finite amount of time to spend looking at their phone per day. The more time they spend scrolling Facebook, the less time they spend on other apps. The less time spent on other apps, the less money they spend on IAPs.
If ads start bringing in less revenue, more developers will move to IAP (or other stream of revenues).