We can't tell them what they should do about discounts or promo's just because they are accepting bitcoin as a payment. It's already a privilege if ever most of these merchants you'll talk to going to start accepting and adopting bitcoin as payment. But that's just it, we can't do something about it if they don't have the nerve to adopt it. If they see it profitable and helpful to their business, they'll adopt it but if there's no use to it and their customers aren't aware of bitcoin, they won't use it.
In addition, the transaction volume processed by payment gateways is also decreasing.
I don't think so that the volume is decreasing. $5B daily volume is too low for you?