Delta certainly needed the Pacific operations that Northwest possessed, and DL got three NW hubs in the deal, DTW, MSP and MEM. DL has added a lot at DTW, a hub that doesn't have a ton of O&D but is a perfect connecting hub (not unlike PIT). It has great geography and very nice airport infrastructure. By all accounts, DTW has been a great addition. MSP, not quite so much, as it has not received the same level of increased service - and may have seen some cuts. MEM? It got a taste of what happened at CVG (and what happened at STL). Overall, however, DL got some great real estate and plenty of customers. NW had some LGA slots that, when added to the Doug Parker gift of lots of LGA slots, makes DL the dominant carrier at LGA.
United gained a huge hub at IAH when it combined with CO, not unlike AA's DFW hub. IAH ain't going anywhere. EWR? I laughed when Burr and PeoplExpress talked about the new EWR terminal over 25 years ago, but it's worked out for CO, and finally, UA has some strength in the NYC market again, after watching from the sidelines and trying to connect the UA faithful to IAD for international flights. CLE? Long ago it got the CVG/STL/MEM treatment.
US? If PHX could support significant numbers of international flights to Europe or Asia, someone would already be flying them. US isn't, and a combined US-AA ain't gonna fly them either. Yes, there's a flight to London and maybe a couple others, but LAX and SFO will be the Pacific gateways, not PHX. CLT? Takes a lot of feed to support its flights to Europe. Those people are connecting anyway, so why not connect at MIA or JFK? PHL? Same thing. If you're connecting at PHL to Europe, why not connect at JFK or MIA? That just leaves the CLT and PHL O&D traffic to Europe, and they might not want to connect at JFK or MIA - if their nonstops cease to exist, they might prefer to connect at EWR or ATL or IAD.
Others have said it better than me, but the plan seems to be "merge and then use US to feed the AA hubs." if US does that, how will it keep feeding the US hubs?