I don't know how that helps. PayPal and stripe are the payment processors, no? Visa and MasterCard are the payment network. Steam can build their own Stripe probably, but are they going to make their own credit card network too? Probably not. They can maybe try taking money directly from your bank though, like Wise. But if you've ever tried it, that looks like such a shit show. Every bank and every country does it a little differently, has its own limits and fees, and the authentication is really ghetto.