As a user, if I go to buy something in an app and it asks me to punch in my credit card number or sign in to some payment processor instead of just the usual "double click to apple pay", there is a very high chance i will just change my mind.
A 30% discount changes things, though for most developers it would be closer to a 10% discount, since they normally only pay Apple 15% and still need to cover the 2-5% their alternative payment processor charges. If it's a recurring payment (i.e. subscription), I would still be inclined to pay the full Apple price simply because they make subscription management much easier than anybody else.
