Amex was the party who voluntarily walked away from this one though that's not what everyone focuses on. The card was already marginally profitable for Amex before Costco asked for lower to no discount. For issuers which rely more on revolvers for their revenue stream, this might make sense, but for an issuer like Amex which relies on discount from spend far more than interest on unpaid balances for its revenue stream, it's not surprising to me they opted to walk away. I would also not be surprised to see Delta and Amex part ways as Amex is paying Delta way more for Skymiles than Skymiles are worth.