I must ask why do you feel that the UA/CO combination is better that UA/US???
CO has EWR, and US has PHL. US is the only one that can offer a Southeastern Hub to the UA network, which is an area they have very litte presense in. 😉
I agree with R U DCA on most merger potential scenarios so I'll toss in my 2 cents...
1. EWR is far more valuable than PHL. CO's EWR operation is the largest hub operation in the top air travel market in the world. Plus EWR is a little further away from IAD.
2. IAH is far more valuable than CLT.
3. CO Micronesia would fit in very nicely with UA's west coast hubs, Hawaii, Asian and Australian network.
4. CO has a presence to South America, which is another weak spot for UA.
5. While both US and CO have a widebody shortage, CO has more widebodies.
6. CO has a presence in unique international markets (TLV, India) that UA does not have a presence in.
7. CO would probably require only a small part of the airline to be divested by the DOJ, as there is little overlap. With US, there is the whole issue of IAD/DCA, PHX/LAS/LAX/SFO. I can't see UA wanting to operate PHX and LAS hubs.
8. CO has done a better job of attracting and keeping high-yield business passengers, which is also UA's business plan. US' business plan appears to be to become an airline that is all things to all people, which is not what UA wants to do.
9. While the Airbus and 737 narrowbodies match nicely between US and UA, the reduced commonality between UA and CO isn't a dealbreaker IMHO.
UA has a great route network which only has holes in the Northeast (specifically NYC), deep south and south central (Texas). Merging with CO fills two of those holes. If you could just slap a couple of hubs on to UA's network, you'd want one in NYC, ATL and either DFW or IAH IMHO. CO gives UA two of those.