and you expected that either AS or DL would make a different decision?
whether AS realized it or not, DL had options to ensure it had feed and they have the same options with AA as they did with DL regarding serving as a codeshare partner, and, despite all the hoopla about AS and DL, there is still a long-term contract with performance requirements that won't go away unless one side or the other decides to buy its way out of the contract or the two decide to mutually terminate the contract - and based on what has happened so far, the cost DL is likely to demand is gates at key airports on the west coast.
And none of that affects what happens with AA and AS since there is nothing that I have ever read that prevents AA and AS from pushing their relationship even further; other than putting the AS code on some of US' network, there is little additional benefit that is going to come from the AA-AS relationship.
cutting DL in favor of growing a relationship with AS will cost AS money.
perhaps at some point, AS will decide it is worth it to pay DL to leave and then rebuild without their revenue.