MANFOD
I think that was partially what I had in mind. The issue seems to me to be whether the MAN-JFK/EWR O&D market (i.e. Non-connecting PAX) is strong enough to demand a "late" service.
The jump in 2014-2015 figures seems to be a good indication that it may be. However the wider figures for MAN-USA traffic for this period indicates that overall growth is not as robust. Do these flights attract connecting passengers? If so where do they go to or originate from?
MAN "suffers" from being very well connected to the hubs of LHR, AMS and CDG. Using your example of a west coast transfer, passengers have the option of routing via these hubs on BA, KL, AF respectively (I have omitted to include DUB &EI here but I'm sure the point is made). These routings also have the additional advantage of likely being on a more modern product than the B757 and B767 that operate ex MAN and whatever service is on the internal US flight.
Then there is the often mooted possibility of a direct west coast service from a "major" airline. How would this affect services like JFK or ATL - both of which are currently very successful.
I'd like to think the general demand is there, but when you have significant momentum of funnelling passengers in these routes on frequent flights via LHR in particular, it is difficult to overcome that inertia without significant effort. Whether this can be achieved by one or more of the U.S. airlines or VS is at best uncertain.