Why can't both statements be true? If AA can't make enough money to justify certain BOS routes but JetBlue can, leading to JetBlue "taking over" certain AA frequencies, then JetBlue is in a sense forcing AA out.

I think we could have a lengthy (and probably inconclusive!) argument about whether AA leaves routes and JetBlue fills the void, or whether it's really JetBlue forcing AA out of routes by being able to profit on routes that AA can't profit enough on, but I'd definitely say it's safe to say that JetBlue is "encroaching" on AA at JFK and BOS.