I've found that they have done just that at PHL. It wasn't easy to knock US off balance a few years ago and that's exactly what happened - on time, generally more friendly and they didn't manage to lose your luggage. During IRROPS (poor weather, mechanical problem, etc), its a lot easier to sub a 737 for a 737 than have to find a way to cram an A320's worth of people on an E170 or two.

Granted, the dynamics of PHL are a little different from the NYC airports, but Southwest seems to be effectively penetrating BOS.