Speaking for myself at least, I've purchased quite a few mobile apps over the years. If I need an app in a certain category, I will always get one that costs money over one that's free with ads. I'm sure I'm in the minority, but people like me don't have to be the majority for someone to make money from an app. The only thing that matters is the number of people like me out there, not the percentage of total users we represent.
I do think there's much room for improvement of course. I wish they would do stuff like allow people to offer paid upgrades. But claiming that there are literally no good paid apps is just silly.
Apple encouraged a market of many cheap-as-in-shoddy $.99 apps instead of inexpensive yet useful and reliable software.
So basically they conned people into a walled garden without providing any of the benefits they explicitly(or implicitly) promised.
As long as they get 30% of fart apps, who cares, right?
How many apps have you downloaded and how many are actually useful to you?