There is no evidence that anyone was better off with higher charges.
Amex has a different business model to Mastercard and Visa, it charges fees and gives rewards, introducing a cap immediately, would have damaged Amex substantially, so it was given three years to make required changes so it could abide by the cap and other regulations. Amex decided that the new regulations would not be financially viable as it would change how it operates its business,
Because a retailer would have a direct relationship with Amex it charged higher fees which meant a lot of retailers just didn’t sign up to it. However some banks offer Amex acceptance at a lower rate than going direct with Amex, that’s why recently more retailers accept Amex, but that then brings in another party thus fee capping.
In the US Amex is cutting its fees to get a bigger share of its core market. It is also subject to possible regulation in the US.
It’s fees are just not seen as fair or competitive. That is partly why its no longer that bothered about the EU, it makes most of it’s money in the American market, so needs to focus on that or go bust.