Yup. I too regretted for United its failure to capitalize on those opportunities. However, keep in mind, United was already growing astronomically during Wolf's tenure. I don't think it would have had the financial resources to inherit even ATL or EA's Latin America routes singularly. Even during Wolf's worldwide UA roundout, UA incurred its heaviest losses. That was the price they had to pay for a truly global network.
I totally agree that UA is almost nothing in the SE US. It's not very strong in the Texas region either, where AA, CO, and DL have formidable operations.
However, every airline has its weakness. AA still isn't all that and a bag of chips on the west and east coast, even though it's acquired Reno Air and TW's eastern presence.
DL and AA have strong domestic coverage, but I'd say its a toss-up between the Big Three. United, while lacking in the SE, has a domestic hub system very conducive to business travel, with hubs located linearly across the central longitude of the country. This allows for a productive amount of efficient connections.
I'm gonna go out on a limb here and say I don't think there will ever be a true national carrier, as the merger between US and UA was envisioned.
As carriers get larger in size, costs become astronomical. I'll say it once, I'll say it again: there are no economies of scale in the airline industry for megacarriers.