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.
Would the iPhone be as successful if it took a 99% cut and nobody would bother developing paid apps for it? Probably not. Whether 30% is the sweet spot is unclear, but given that there is plenty of paid apps on the platform, why would it be unreasonable?
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.