Unfortunately, that's not enough to shake the MasterCard/Visa stranglehold. Even if all of Valve's German customers used Girocard and Steam sold those particular games only in Germany, they would still have to yield to pressure from MC and Visa because losing them would cost them many more of their global customers.

It's not enough to simply have an alternative to the credit cards, that alternative has to be in the pockets of 90% of your user base before you'd be willing to lose the method of transaction they currently rely on.