1) apps that you pay for, either up front or free but then via IAP, but STILL have ads 2) Ad cannot be removed by paying IAP. They're either still around, even if in part? Note, this isn't whether you like this or not, but more so how that trend as gone. I recall with Angry Birds 2, there's no way to pay some "premium price" ($5, $20, $40, whatever) to unlock the game as if it were a traditional, premium game, and there appears to be quite a few games like this nowadays.
I don't think it makes sense to keep showing ads if a user already paid. That just drives users away. Either doing F2P and allowing upgrades with IAPs, or paid apps. That's the way to go. And if you are keeping your app free, then make sure you get the best ad network or mediation. What ad network is currently working better for you?
