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.
The issue is Visa/Mastercard/whoever is pressuring Valve isn't happy about the very existence of incest games. They don't want to be associated with incest/rape even indirectly.
If a book publisher was selling erotic fiction about children online, you could bet your ass they would have a hard time with payment processors.
I’m not sure you have a case with this argument.