BHX to USA
Looking at the raw passenger numbers, it is admirable that AA are intending to stick with BHX/JFK through the winter.
Looking at the seat maps on the AA website the pattern looks clear; good sales in the cheap seats, poor uptake in the premium economy seats, and poor take up in business. Of course it is holiday time on both side of the atlantic, so perhaps the low business take up is perhaps understandable, and the proof of that pudding will be September / October.
Taking both UA and AA into consideration, you could conclude that regional flying to the USA, that is apart from MAN and EDI, just isn't viable at any significant capacity, I guess partly because people would rather drive to LHR / MAN and take a direct flight to the final destination, than fly from the local airport and hub in JFK or EWR, especially as there is no real cost saving to be made. I hope my analysis is flawed, and that BHX (and NCL) keep their UA / AA services into 2016 and beyond.
For whatever reason the same logic doesn't apply to passengers heading east, and however much kudos is gained from chasing direct USA services, the concentration of marketing efforts ought to be more towards the (until the last few months) developing markets in Asia.