With all good things, there is a point of diminishing returns. An airline can grow to:
1) Increase scale, gaining efficiencies. This maximum hits pretty early by US airline standards. VX is probablynearly there.
2) Increase scope, adding new city pairs. The old US and HP didn't have full domestic coverage, but the merged US does. AA mostly does, though they've gone in and out of the domestic coastal north-south markets. Internationally, AA is strong to the south and east, but not the west.
3) Reduce average labor costs by reducing the average number of years of service. This tops out pretty quickly. B6 and VX may be the only two for whom this strategy still has decent legs.
Once you get scope fully covered, and topped out on the other two, mergers in and of themselves dont buy you anything, except perhaps a bit of time if you can manage a quick drawdown of capacity. Even then, it doesn't take long before someone with lower costs comes in to soak up that drawdown.
Of the legacy carriers, AA and CO appear to be the healthiest, though in different ways and for different reasons. Both of them can gain some by increasing scope, and perhaps one or both of them would do so if a combined UA/US drew down in a strategically useful area. I could imagine AA or CO being interested in CLT, for example.