It's pretty questionable whether NW and CO would have the kind of long range heavy pax trunk routes that make A380 a runner. Just because they haven't shouldn't be carte blanche to slag it off because not many folks want to fly into DTW and EWR comparatively speaking.
NW and CO are good 777/7E7 candidates and good luck to them, but if they were able to fill enough seats to get 380s they would buy.