There are always two limits, what producing something costs and how much people value the thing and are therefore willing to pay for it. The actual prize must be somewhere in that range. A thing is a success because the prize is less than what people are willing to pay but they might still be overpaying if a lack of competition does not drive the prize down towards the costs of production.

That seems like a false dichotomy. Anyway, it's unprovable either true or false, because no evidence exists.

Let's just assert that it's false, then. Let's say 30% is unreasonable, and that iPhones would be as successful even with yet more unreasonable developer fees.

Well the iPhone is a worse device than a flagship android already, so I think it would still remain popular because of the fashion aspect of being an apple user