I used Amex before DL and will likely to continue to use them after DL. I have a Visa for backup, but I have not had to use it in ages.
I would vote with my wallet, but options for my flight dollar are limited - NW/DL is usually the cheapest fare for my travel, especially when you factor in bag fees on the discount carriers where I wouldn't get enough travel on any of them to get elite there anyway.
BUT... looks like EAU is going from DL via Mesaba to MSP to United via Skywest to ORD, so that may open up a switch, or I might just go with a split between DL and United based on which fare is cheaper that day.
All those rollover miles will make it easy to give UA a whirl and still come back if I want without any hit on status.
Where DL is going to lose my money is no more Skyclub membership when that expires, no more preference for booking my staff on NW/DL, and likely no more money from my Amex card (switch back to cash back or member rewards).