A B6 merger would never float with the fed, too much concentration in LGA, JFK and BOS, would have to divest major portions.
What? I'm pretty certain, they would be smaller at the combined LGA/JFK than DL, though it would probably be close. BOS would definitely be the largest as jetBlue is already the largest, but a decent portion of those routes are duplicate that could be eliminated.
Ideally, BOS becomes the international/Europe reliever airport to PHL and CLT routes are to South America, and routes that can be supported by local demand, and connecting traffic originating in the south.
BOS flights to CDG, Hearthrow, TLV, FCO, FRA, and Dublin all make sense if they stay in Star. The trouble at BOS is getting MassPORT to help get customs at C, given the futile efforts at getting them at A.
JFK could be code-share on any partner airlines and would not need US to fly international. JFK is obviously already profitable for jetBlue, US's move would be more to control the flow of passengers, than to expand internationally from there. (funneling them onto a code-share partners and getting 1 leg is better than UA or DL getting both legs.)
Again, even if they have to divest some routes, the opportunity to be #1 at BOS, PHL, DCA, CLT, and #3 in the NY and Florida-as-a-whole markets should be a compelling reason to do the deal. The only hole on the east coast is Atlanta. I think its very underestimated the power of that link-up.
Let's face it, they have a high-cost model, no matter what they call themselves, and if they can't get costs down further, there is no reason to fly western routes except for the high-yield routes and glamour routes. The PHX hub can no longer be profitable given the current corporate CASM.