Well I guess AA is untouchable and should not be messed with. The are a force not to be reckoned with. Someone has to compete.... I'd like to see US go head to head and not be a follower for a change. US has always been known to react instead of ACT.
I don't think that AA is untouchable at DFW or anywhere else. But not too many legacy airlines have ever taken on an established fortress hub airline at their hub. Low-fare (maybe low-cost) upstarts have done so, like Airtran at ATL or Spirit at DTW. WN has moved in when it smelled blood in the water, but it offers much lower costs and a complete absense of BlowFares/ExtortionateFares.
As WN and B6 continue to expand quickly (even with the announced slowdown by B6, its expansion is still huge), someone needs to pull down capacity, so growth is really a fantasy for all the legacies. They can re-allocate some domestic ASMs to international flights to try to hide from WN and B6, but that's not expansion. UA, NW, DL and US all contracted last year, offering fewer total ASMs than in 2004. My guess is that many legacies will continue to shink.
If you're slowly shrinking, you really can't move into established hub cities in any meaningful way, unless you want to spend lots and lots of borrowed cash.
AMR is not invincible, but I don't see UA, DL, CO, NW or US picking any fights with AA right now. Nor do I see AA picking fights with any other legacies.
I do predict that AA might finally turn the screws on B6, but I've often been wrong before.