The prices that JB is charging now might be low, but that's likely to stimulate the market and not sustainable in the medium to long term. I bet by next summer the prices will be higher, esp for Mint. Also, starting this month JB had to reduce service to 4x a week, while starting service to Gatwick (4x a week too). Business travelers don't like service that isn't daily. Now they say they'll increase service in the future. But any future growth will need more slots, which are becoming expensive again (from Avweek).

Fuel, landings fees, ATC service, passenger handling fees are all the same. Aircraft acquistion cost is close - the only variable is labor and overhead. One thing the legacies can do is spread a lower profit on a BOS-LHR market over their entire network - so while they are losing $$, compared to the whole network it isn't a lot. And with Delta building up BOS with service to non-hub markets, it won't be long before we see LHR-BOS-CLE or LHR-BOS-DEN (or whatever), competing with JB for the whole trip.

In the end, the wide bodies (lower cost per seat mile) and large network the legacies provide I think can match the "cool" factor of JB and the fresh service they provide.