Some additional context to my previous comment: I work tech in the financial industry. I have some experience with payment processors and the stupid amount of bullshit around all this stuff. "They could do both of these things almost instantly" is a big assumption holding the entirety of the weight for your argument.
Anyway.
50 Cent was doing a one off publicity stunt, not trying to ensure continued existence as a company. How many mainstream artists are still doing that? I shouldn't have to say that this is very much an apples and oranges comparison.
Your other idea has legs, but it's still suggesting that Valve try entering a game of financial chicken with Visa and Mastercard. Effectively infinite money. And in the meantime most users would just be pissed off at Valve for making it harder to buy anything. We're already seeing people attack itch.io for not standing up instead of bei g pissed at the payment processors.
Valve can't make purchasing through a different processor a requirement for some games but not others because Visa and Mastercard said "stop selling games with this content entirely, or we stop processing your transactions entirely". So anything they do will have to effect all transactions.
I'm frustrated Valve didn't do more, and that they've not made any public statements about trying to fight this, but Valve isn't just leaving money on the table because they're lazy or dumb.