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 can’t run your own card network easily because you would have to convince all of the merchant banks that take card transactions to do business with you.
Digital money movement requires an operating agreement between at least two financial entities - but most of the time there’s a lot more. Depending on the type of transaction you may have two or more gateways, facilitators, processors, issuers and underlying banks involved.
It’s a very fragmented system that relies on many, many different entities all having agreements and contracts with eachother.