I hope to be proven wrong but I don’t think bonus category makes any financial sense as the AF goes up.

They will barely break even on 2x categories and most people are smart enough to focus on bonused categories for their spending. So in effect, they are likely making more money on AF than on fees collected from spending for a vast majority of their customers who care about bonus categories.

So the challenge for Amex is to get as many people sign up and keep Platinum as possible, even if they don’t spend a penny on it.