Even if the percentage overlap approaches 90%, I don't think it's a smart move. It's very anti-customer and will cause a decent percentage of people a great deal of angst and a much larger percentage of people will be quite disappointed. And don't forget that high-spending corporate travelers use Amex almost exclusively.
The problem with a payment provider striking an exclusive deal like this is that it creates very real problems for people unlike, say, and exclusive beer deal. And it creates a great deal of negative feelings towards both Visa and the Olympics. I'm sure Visa is able to show that it is a good deal for them. But I suspect it is just as easy to show the opposite. Not smart at all.
The problem with a payment provider striking an exclusive deal like this is that it creates very real problems for people unlike, say, and exclusive beer deal. And it creates a great deal of negative feelings towards both Visa and the Olympics. I'm sure Visa is able to show that it is a good deal for them. But I suspect it is just as easy to show the opposite. Not smart at all.