Good news! Southwest's entry into BOS will certainly lower the US fares on the east coast. If we're lucky, maybe even strike a fatal blow against US. US is moving in like a predator looking for a kill.
Time for AA to pack its bags and run? From WN? Like AA did at LAX? Or IND? Or MCI? Or any other WN city? WN has certainly killed AA's business in Chicago and Dallas. Imagine what those hubs would be like if Southwest didn't exist. Maybe AA would even fly to a foreign city or two from those hubs if not for all the damage WN has done to AA in those cities.
If anyone should be worried (besides US), it's jetBlue. They've been expanding their base of operations at BOS and this should temper those expansion plans.