Surely a service to Los Angeles would be more popular and convenient for people to connect to the U.S West Coast, to places like San Francisco, Las Vegas, Hawaii & many others. Since the airline is well known in the US it will bring in a lot more U.S originating traffic, but also because it is known internationally and has close ties with BA, it will also capture UK originating traffic and maybe even traffic connecting at MAN. There has been growth in AA LAX long-haul operations with the 787-8 which would be a perfect aircraft for this route.
I don't really see how if Chicago cannot even sustain a year-round service and demand is not that strong, how a route to Dallas is going to work. I know it is an AA hub but connections to other Western and Central American destination s can be accessed through the New York and Chicago hubs.
New York is the only one performing well as you said, with Chicago down 23% in 2015 and Philadelphia down 11%.