Thanks for your input. I agree this could be a risk. Especially for Amex specifically, where people shun all things American and Amex's brand even has "American" in it! I'm taking a long term view here in which I believe rationality will prevail over irrationality and the current protectionist policies will be abandoned in time - globalization and trade has brought so much prosperity over the last 80+ years. Over time the economy will grow and the volume of goods and services will grow and Amex can capture a slice of that. Also, Amex has such a good product / business - they offer higher perks and rewards so that people want their cards and then they charge merchants even more than the other companies and retailers feel the pressure to accept it. I'm in Canada and I have an Amex card, my spouse has one and a lot of my friends do as well.
When I looked recently Visa's P/E was 34 and MC's 39 so rather pricey still if the growth doesn't continue. Amex's P/E is at roughly 17-18x.