Maybe the rot set in when they lost Costco ...
"Costco Cards Account for One Out of Every 10 AmEx Cards in Circulation" - WSJ
https://www.bloomberg.com/features/2015-how-amex-lost-costco/
https://www.fool.com/investing/2016/06/22/how-bad-management-led-to-american-express-costco.aspx
http://viewfromthewing.boardingarea.com/2015/10/15/an-inside-account-of-how-amex-lost-the-costco-deal-and-what-it-means-going-forward/