Practically impossible.
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.
Why does ValveCard need to work anywhere other than Steam? Privacy.com manages to issue card numbers somehow. How does that work?
Entirely possible if you're JP Morgan Chase. They're big enough to have both merchants and consumers in their ecosystem, and they tried it, and Visa put a stop to that.
What if you used your mastercard to buy valvebucks you spend on whatever the hell you want in the steam store?