Think about the situation for a second. You'll be switching to other methods, and so will everyone else, all at once. Do Amex and Mastercard even have the capacity to take up the slack with no notice? Do you really think that every critical component of a cost-optimised system will have been over-provisioned to be able to take on hundreds of millions of additional customers in a single day? And if one fails, the demand switches to the remainder and crashes them too. It's a classic cascade failure.