they had hundreds of thousands of employees, and they didn't need to deal with all the digital shit we gotta deal with now.
They needed hundreds of thousands of employees because they didn't have "digital shit". Today, the entirety of Discover Financial Services is around 21k, and probably falling.
If Valve did it, it wouldn't be under the Valve organization anyways. It would be a subsidiary, and Valve has plenty of cash-flow to build it out.
What payment terminals? They could go years just being an online credit card. Hell, initially it wouldn't be very different from any company that bills their customers. Start it as a Steam only thing, then add select partners one at a time. It doesn't have to be in your grocery store on day one, or ever really. Fraud detection is easy when you can just yank the game back. Sears couldn't do that when you bought a washing machine. I worked in banking infosec and I have no idea what "digital compliance" means in this context. The hardest compliance standards in this space are PCI, and those are defined and enforced on clients by the payment card industry itself.
Which is why I specifically said it would be run as a subsidiary.
Gosh, where on Earth could they find people with experience running a company that would look like 99% of the companies in existence?
You're just throwing shit on the wall and hoping something sticks. You could neigh say anything, and nothing in the world would ever be accomplished.