I'd be interesting to know if Valve is big enough to start their own payment system. Yea, I know it would be hard but their customers have libraries of games in their system and Valve has lots of good will. Valve could also offer discounts ($X off if you use ValvePay). It would take years. They'd have to drop the adult games now, start ValvePay, promote it until the majority of their customers used it. Then put the games back and tell Visa and MC they can eff-off.