That's my worry. In a feeding frenzy like this, somebody has be the first airline to "go too far" and oversaturate the market. Somebody, at some point, will be last guy to show up at the end of a bubble with a tray of tulips and not know what hit 'em.
Where is all this traffic coming from? How much is stimulation (or release of pent up demand) vs straight up stolen from old school connections via LHR, CDG and FRA?
Is Boston (as an airport and as a business market and as a tourist origin (destination?)) doing what it has to do to keep/support all this growth?
Apart from new stimulation, stealing passengers who would normally have flown on other airlines, and the JetBlue factor, I am going to assume there are people choosing the non-stop out of Boston now that there's one available, rather than flying out of an airport that's closer/easier for them. Example, my friend's father has to travel to China multiple times a year. He, and most of his team who also are sent to China multiple times a year, live closer to Providence. Before he would always use Green because it is easier for him and he knew even if he flew out of Boston he would still have to connect. Now he can drive a little further to Boston and get a non-stop flight to Beijing, instead of connecting via JFK, or O'hare as he used to do.