But that's how you get from one to the other. By refusing to use something else, the developer would have to sell through Apple in order to get your business. If they want your business, that's what they'll do.

Meanwhile other people may choose to do something else, e.g. because they would rather that more of what they pay go to the developer (where it's plausible that it be used to improve the software) than to Apple (which can't productively use any more money than it already has). And who are you or Apple to get in the way of this transaction between two independent and consenting third parties?