To replace visa/mastercard you need to have thousands of banks support ValveCard across the world. It's hard to imagine how it's going to happen. Players will not switch to another (probably foreign) bank just to buy Half-Life 3. They'll pirate it.
By the way, Gabe has a very famous quote:
> Piracy is a service problem.
He knows it very well that if it's hard for players to buy something they'll just get it free anyway. You can say he's probably the first person in the world who realized this idea profoundly enough to turn it into a business. It's very risky for Steam to make buying games even slightly harder.
You get your card from your issuing bank, so the consumer’s last mile is the bank’s problem. The merchant get their POS/gateway from the acquirers. Your bank and the merchants acquirer don’t know each other.
Visa and Mastercard are intermediaries. There’s no way a NatWest card in the UK is connected to whatever POS is in Chile or whatever. They all route through the card brands.
This is why it’s so tough to break this monopoly.