DL has not been a "standalone" entity since it merged with NW shortly after emerging from bankruptcy. Therefore, all history to date proves is the combined NW+DL has fared quite well. There aren't facts to support any other conclusion.
DL emerged from BK and was a standalone company from May 2007 until the NW merger closed late in 2008.
Once again, by the time the DL-NW, DL showed an operating profit minus merger/restructuring charges while AA, UA, and UA posted operating losses. DL did in fact show better results post BK and did it while still a standalone company.
Since the merger, DL has accelerated its earning performance. Over the past year since its own merger, UA/CO has posted very similar financial performance to DL.
I am not arguing that there isn't a merger premium that doesn't come from earnings... I am saying that the DL and UA mergers were structurally different from what AA/US could be - and it is still doubtful that AA/US could climb into the same league as DL or UA because AA/US still will have a very small Asia/Pacific presence, will be very weak in continental Europe, AA's historic strength in LHR has been neutralized by the UA/CO merger now that UA/CO is about the same size as AA on its own metal while DL is doing very well in LHR in the limited markets in which it has enough slots, AA's advantage in Latin America is largely insulated by limited access markets which will move to Open Skies in the next few years, and domestically, AA/US will not be the #1 carrier in any of the top business markets - NYC, CHI, or LAX - and the gap between AA and the #1 carrier in each of those markets - all for now UA - is growing. DL is narrowing its gap w/ UA in NYC and might grow to the same size as a result of the slot deal, Asia Pacific is largely split between DL and UA with DL being dominant in Japan and S. Korea while UA is dominant in China/HKG - among the largest markets.
.
And then you have to consider game theory - what happens if AA/US does merge - still not creating a large enough carrier on the east coast to be dominant in anything outside of their own hubs - while DL and UA then make their own moves which likely will include a DL acquisition of AS which pushes AA/US even further down in the west or DL or UA expands their own MIA-Latin America service. And you still have US to contend w/ which is focusing its growth on Texas - DAL and HOU - markets that will affect AA and to a less extent UA (even though they act like it would be the end of the world) far more than they are growing in DL strength markets.
.
There is a first mover advantage in business and DL's completed merger with NW as the first of the megacarrier mergers gives DL a larger advantage in making the next move in consolidation. UA moved next and is largely executing its merger well - although the real question is what their costs are going to look like after they give pay raises to buy labor peace. WN's merger will work out and they will be stronger in time. All of those mergers will be finished long before AA/US even get the paperwork signed, let alone begin to start working as one carrier.
Thus, the prospects that AA/US will be able to compete with 3 megacarriers - one nationwide and 2 global - are not real great, esp. since those other 3 all have stronger financials and have network strengths that are more valuable than even what AA/US can come up with.
.
I still think the best chance is for AA to bring its costs as low as it can in order to compete as effectively as it can... but a merger with US will add to costs and delay the ability to compete at a time when AA is already behind the 8 ball compared to other carriers. Throw in that there is labor discord at AA and US and the chances of a smooth merger are slim. Finally, add in that other carriers can satisfy AA creditors NOW and demonstrate stronger returns and it is doubtful that US could really win a merger proposal for AA. Remember, US lost out on merger proposals in the past - and they weren't even competitive with other carriers - and that will certainly be the case with AA.
.
It is also possible that AS or B6 could buy parts of AA in partnership with DL and investment bankers in a way that doesn't create the duplication that US would have to resolve - and AS, B6, and DL all have stronger financials and employee relations.
UA probably could buy US with the exception of probably about half of US' DCA slots which could easily be divided between several other carriers.
.
I think it is more likely that not that eventually that the US airline industry will consolidate around two global megacarriers, one nationwide megacarrier, and one or two other large nationwide carriers.