It means that if those payment processors gain market share from Apple I will need to support them to reach the exact same userbase because if the users adopt using the alternatives it is very likely that they will stop bothering to upkeep their Apple payment methods. It happens all the time and there's even API for it to offer the users a grace period until they fix their payment.
If your customers prefer to use a payment processor that costs you 3% instead of Apple's 15%/30%, then why wouldn't you support that payment processor? You gain more revenue and honor your customers' preferences at the same time.
Because supporting payment processors is extra work(which is requires technical and non technical skills) that can go into development, that's why. It is something that I would prefer to delegate, pay the due and forget about it.
What's so hard to understand that? Have you ever created a product and made money from it?
By migrating from Apple's 30% fee to a payment processor that charges only 3%, your revenue would increase by 38.6%. The only way that additional revenue would not make up for your development cost is if your revenue were low in the first place. There are plenty of developers who would gladly integrate another payment API for the additional revenue.
My revenue wouldn't necessarily increase(unless maybe price reduction drives the sales enough), my costs due to Apple's commission will decrees and my development and legal costs will increase.
My landlord doesn't care. From his perspective, all that matters is that if I make enough money to pay the rent. He is not interested if my revenue after Apple's cut has increased.
If my net income decreases(because my costs increase more that the revenue after Apple's cut increase), I am screwed.
