In the grand scheme of things the LCY-JFK flights provide a grand total of 64 Club seats each way, per day.
Compared to 5 x 747s (350 J seats) and 2 x 777s (96 J seats) from LGW/LHR to JFK each way, per day...so 446 seats and that doesn't include the EWR flights (another 144 J seats).
So the new LCY flights provide about a 10% increase in capacity...hardly an eye-watering increase. Admittedly it's not the best times economically to add capacity (most are reducing) but I imagine that the BarCap deal is the reason why BA are still going ahead.