VS cuts are actually due to problems with the 789 fleet being fixed over the winter period .So the 333 fleet are in demand at LHR along with some Delta 763s in a supporting role!
Delta/Virgin are working very hard to maintain a scheduled at the moment - Perhaps they need to call back a few 340s from the sunshine in southern France !
I have some hope that the VS/DL marriage remains on track in the medium term
As for AA/BA and JFK not too bothered actually - Its been very clear for some time that the eastern connectivity international/domestic would move to Phili with JFK being more point to point and that in the case of the UK in particular that JFK- LHR would cover all and more of that traffic.
Think the desire to retain Chicago a route they pioneered BEFORE even being in Heathrow in any big way is a signal of their desire to remain in the Manchester market longer term and that it may well operate through winter 2018/19.
As for United I also see this staying and even being upgraded to a 763 at some point.