If 50 Cent could sell album for crypto from his nothing website a decade ago I feel like Valve has the technical wherewithal to implement one of 1,000 preexisting checkout solutions in the short term.
I think selling steam giftcards (an existing solution they're already using) at a markdown to expand that business would be pretty viable for a company that regularly marks their products down by up to 90%.
They could literally do both of these almost instantly as preferred options while still accepting the big cards.