No surprise there, four carriers on MAN-New York was never going to be susitanable and I wouldn't be at all surprised if UA are the next to pull out. The more direct options TCX add the less demand there will be for connecting through hubs.
As for talk of BA taking over - the question must be how could they do any better? MAN-JFK is a joint venture so if AA are pulling out they can't be making money. With no base at MAN BA would face the extra overheads of positioning crew, aircraft etc; it just doesn't make commercial sense.
They don't have the right aircraft and if AA can't fill a 757/767 I don't see the recongifured 777s being more successful. Plus they are having a hard enough time battling Norwegian at LGW, to go up against TCX and VS at MAN would be suicide. Frequent flyers will just be funnelled through LHR, ORD or PHL.