I think discussion of whether the Amex Platinum is worth it (which has been debated ad nauseam on other threads) is a separate issue (personally I get a lot of value out of the cards regardless of club access). I think what us more relevant here is from the Delta side - what do they want to do with lounge access, how they want to make money, how they want to position their cards (cobranded and Amex Platinum) versus competitors