I think the trick here will ultimately be to add a layer of indirection. Mindgeek could launch a bazillion internet bodegas that sell gift certificates that happened to be redeemable at Pornhub and their other properties. They could operate their ad business over regular banking networks. Basically anything and everything ti make it more complicated for Visa and Mastercard to simply look up Mindgeek and block the account. It would also provide some relief to Visa/Mastercard since they could say truthfully that they don't do business with Mindgeek. You can also counter the bad propaganda by pointing out that Visa/Mastercard are still very active selling alcohol, tobacco, narcotics, gambling (half the US states allow you to buy lottery tickets with credit cards, all the casinos take credit cards), guns, and pretty much every other vice. Though I don't think their role in purchasing guns makes them accomplices to murder anymore then processing a payment for a porn site makes them an co-conspirator to anything related to that. But if they want to make the case that they are then that sets precedent they are co-conspirators for everything else as well.
