Yes, but if their own ad converts the user to a paying customer, they don't pay 30% of their revenue to Apple.

Yes, but an ad to a competing product would become very expensive, as it wouldn’t convert and it’s cost (cpc) would increase as it is deemed less relevant.

Or the the developer would run their own ad.

But those are very different propositions to a middle man running a ‘genuine’ ad which simply takes a cut.