If delta does move to a southwest style revenue based FF program the vast majority of Delta Amex customers will move to other products...short term most will be ignorant, long term the revenue they now enjoy from AMEX will dry up.
Would Delta even be profitable without selling miles to AMEX? I have serious doubts they could remain profitable long term. I'd love to know the spend & carry rate for the Chase SouthWest visa compared to their other products (BA & UA namely)