> You'd have to onboard hundreds/thousands of banks

It's perhaps a good idea. It's likely that not very many banks and terminal makers and payment processors really matter. It would be a little delicate because the ones that matter would be pressured or at least would feel pressured NOT to participate on threat to their currently main business.

And the project doesn't have to become mainstream probably, just accepted "enough".

A better reason is that it's not really Valve's battle. They have plenty of other business. They don't need to fight this war. A company like OnlyFans, yeah perhaps they do - but they are likely much smaller.

Valve is in a situation that helps: they charge separately for each item. Some that the credit card networks are okay with and some that they are not. So they could support two regimes on their site: some items could only be paid through the Valve new card network (and gift cards and bitcoin), while other items could be paid through all the above plus the legacy credit card networks.

Valve (and/or OnlyFans) then gets paid for trying to enter the very lucrative payment network business. And gets to use these separate charges / two regimes of payments to distribute content that would be too dangerous within the current single payment framework.