They would have to roughly make their own form of PayPal, alongside their own bank.

If you didn't know, PayPal technically isn't a bank, it and Venmo use Synchrony Bank... which is an actual bank.

If they did something like that, it could work, but it would have to be at a similar scale as PayPal, that is to say, massive....

Because doing this would/could basically be the nuclear option:

MC and Visa and PayPal would/could drop them.

So, they'd have to basically develop a massive project, in total secrecy.

... Which is something Valve has arguably done a number of times, they are notoriously opaque as a company.

......

Sort of as you mention, they already have a barebones backend framework to scale up from the steam gift card / user gift card balance system.

I am... uncertain if their backend for that already does or does not include an actual legally defined bank though.

...

Problem is that this would necessitate a massively costly undertaking, as well as ongoing maintenance costs, and Valve is also notorious for basically running on what most other firms would consider a skeleton crew for the size and scope of what they do.