Although I have believed DL/NW was inevitable, I'm not so sure that NW will successfully restructure to be a viable partner. They are having to almost exclusively cut costs in order to restructure instead of also increase revenues through network restructurings as US, UA, and now DL are doing. NW's strategy appears to be to dramatically reduce their costs which will come at the cost of their service levels. Further, other US airlines are growing into NW's Asian markets while NW is forced with using their few new aircraft deliveries to replace gas guzzlers that other airlines grounded a decade or more ago.
Even if NW does successfully restructure, they will be a bottom feeder and not likely to be a long-term player. AA is clearly the carrier that would benefit the most by acquiring NW's Asian rights while dismantling or shrinking much of the rest of NW's operation. Given that most of NW's employees will be outsourced, there is very little incentive to try to keep the operation at the same size as it is today.
I do believe that UA and DL will both restructure successfully although I do believe that UA will be very vulnerable because its costs post-bankruptcy will still be at the high end of the industry average - not a good place for a "new" business to be. Other carriers are certain to pounce on UA as soon as they emerge from BK - and WN isn't even waiting. Also, DL and UA have very little overlap so DOJ considers are muted. In fact, the DL/CO/NW codeshare is bigger than DL/UA even though the codeshare does not accomplish the same thing as a merger.
DL's long-term survival depends on whether it can come out of BK quickly and prove that it is capable of playing in the new environment. Given their business plan, they are aggressively planning to get there. If they establish themselves as having done what's necessary to survive and done it better than most other airlines, they could be an acquirer.
I also think CO is probably most likely to remain alone if they do not find the right merger partner. UA certainly looks attractive to them but UA is still a large airline with lots of obligations and fairly high costs. Reworking UA again to make it work w/ CO is not necessarily an easy process. And all of UA's current creditors will expect to be paid for the investment they are making in turning UA around. UA won't be cheap to acquire unless they start to slide and their restructuring proves insufficient in a couple years. At that point, DL could be as well positioned to acquire UA as CO, whose balance sheet is stressed and only getting moreso and whose costs are also moving toward the top of the industry while facing new low cost carrier competition.
Conversely, UA's new owners are going to be very reluctant to allow UA to buy anything unless it is pretty certain the other carrier can be easily integrated into UA - which begs the question of why that carrier and its creditors would choose to sell itself.
AA is clearly the best positioned and will continue to do what's necessary to remain in control of its financial situation. After that, it's hard to predict too far down the road but I am more comfortable in saying that DL will be a long-term survivor or will be acquired in a pretty healthy form. UA can be the same but it has to get its costs down lower since DL and NW will both aggressively lower their costs in BK. The new U could be an acquirer of NW but probably only if NW falls dramatically at which point AA probalby has a greater ability to acquire them. CO is a likely survivor but they do not have the financial resources of AA to acquire so anyone they merge with will have to be pretty healthy coming into a merger as a result of bankruptcy - at which point there is little incentive for the other airline to sell.
So, I predict that NW will not survive long-term but I'm not sure anyone can be certain how the rest of the pack will shake out. Foreign ownership could change the outcome but I still think US laws will favor a domestic merger if that is in the best interest of US airlines. I believe as a country we have to give US airlines first opportunity to merge if that is best for both airlines before we open US airlines up to foreign ownership, which probably will only prolong excess capacity in the domestic marketplace.