I think what has happened to the IND-DEN route over the past six years is a pretty good indicator of the problem United has in this regard. (This started well before the merger, when IND became a UAX station.) This route has dwindled from 2 to 3 737's/day five years ago to 2 to 3 170's, 145's, or 200's/day. Southwest typically flies 3 737's/day on the route and Frontier flies 2 319's and 1 190 (currently). For me, the choice is pretty clear if I'm just going to Denver--there's little reason to fly a cramped small plane on United, when I can go for fewer $'s and in more comfort on Frontier. If I'm going to the west coast, I'll stay with United and either go through IAH or ORD, on a ERJ170 or larger.

Southwest and Frontier have made this route "work", but United can't (or it doesn't fit in their business strategy).