American Express hasn’t even lost Costco charges yet, and it’s reeling. Here’s the inside story on how Amex lost the Costco deal. Delta is their second largest co-brand, but with Marriott buying Starwood they presumably lose the ability to issue Starwood co-brands at some point when the programs are expected to merge.
American Express lost the JetBlue co-brand to Barclays though it’s small and they will actually manage the backend for Barclays through their LoyaltyEdge business. Their network is no longer used for Fidelity card transactions.