I think Visa will back down. They need Amazon more than Amazon need them.
I don't think that Visa's charges are any worse than Mastercard's - and certainly a lot less than American Express - so this is just a ruse to try to get people to take out Amazon's own Mastercard product.
It's pretty grubby really, the indirect result of this is that people will take out another credit card when they don't really need one, more people will end up in debt as a result of this decision. Not that that is Amazon's problem, as long as they get the money it doesn't really matter.
I've never had a Mastercard before, all my credit cards have always been Visa ones.