> Does the argument require us to believe the fee would still be 30% if iPhone had practically zero users? Or is that legally irrelevant assuming that they aren't found a monopoly?

While not zero users, the default fee has been set at 30% since the beginning of the App Store, long before Apple had the level of market dominance it has today.

This argues in favor of it being acceptable: developers were willing to accept that fee without monopoly pressure being applied, since nobody has successfully argued that Apple held a monopoly in 2008.