The issue is more that (a) debit routing may be problematic (at least from a UX perspective), potentially forcing all debit transactions over Visa/MC and (b) contactless could very well increase card use for smaller purchases like it has elsewhere. Building retailer specific apps has the possibility of eventually pushing a significant number of transactions off the card rails entirely, significantly lowering costs. And if it doesn't, at least the shift away from cash will have taken longer than it would have otherwise.

Also, when you're of the view that almost any amount is too much to pay for card processing, that 0.15% or so that the issuer pays Apple is effectively the same as if you had paid Apple directly. After all, if the issuers can afford to shell out the money for Apple Pay support, they can afford to reduce processing costs by at least that much.