I'd bet that there are more mastercard users who haven't even heard about Steam or Valve than there's users on Steam. And MC with others are using their massive leverage right now, what you're going to switch to?

Also 3 times bigger is definetly something. It's like asking what's the difference between a million dollars and a billion dollars (answer is about billion dollars). Not quite, but effectively the same. And payment processors have the power to practically stop all money transfers to Valve (which they are threatening with already). Microsoft or Apple might have cash to fight that, Valve most definetly could not sustain legal fight with them if absolute majority of their income is cut off.