I think eventually ATI will be granted, it will come at the expense of JFK-LHR frequencies. AA and BA, under the old bilateral, were part of a regulatory quadropoly, which they are benefitting from to this day, due to the scarcity of LHR slot pairs. VS doesnt have much of a leg to stand on because they were part of the same quadropoly. In my mind the only really valid objection to the tie up would come from DL and Skyteam, who were frozen out of the bilateral, and are really the only ones in a position to take advantage of a 'forced sale' of JFK-LHR flights. VS simply doesn' have the A/C uplift, UA gave up JFK-LHR a long time ago. There is no other US based airline with a sizable presence at JFK (leaving aside B6, who don't do international service currently), and given BMI's current state, and lack of sufficient long haul A/C, I can't see them enterring the market either.

MIA and DFW are ridiculous arguments as they represent hub-to-hub traffic for both AA and BA.