From a network standpoint BOS is probably the most in danger. No one airline has a definitive advantage in BOS. DL is adding small markets that US had. The current facilites are not used to their maximum potential. FL and B6 are adding service.
LGA and DCA as slot controlled airports are inherently valuable and LGA has more destinations. US has an enormous amount of pricing and network power in CLT, PHL, and DCA.
The only silver lining is that AA has cut back and DL never came in with a large expansion.
The major hamstring remains the CRJ contracts. The current management has been very vocal in stating that there are too many. As soon as they are up the express fleet will shrink again and a lot of BOS cities are CRJ routes. It is a forgone conclusion that CLT, PHL, DCA and LGA will get the 70 and 90 seaters.
US will never give up LGA and DCA slots so they will fly a kite to keep them active.
Mark my words, BOS is now the weakest link. 🙁