F9 retreating from DEN seems to be benefiting WN more than it is UA. DEN is a perfect example of a hub that needs more mainline planes that UA simply doesn't have. DEN-DTW/GRR/CLT/RDU and frequencies on many others lost mainline when UA retired the 737s. If they weren't so gung-ho on retiring the 757-222s and used those new 737s for growth, they could really turn the tables in DEN and ORD. But that would make too much sense.