Josh,
indeed DL had a strong presence in the BOS-FLA markets for years as did US. However, DL realized it couldn't try to regain its position in both NYC and BOS at the same time and decided on NYC as the market that matters the most.
Fast forward to today and this summer and DL is with a few hundred seats per day from JFK to N. America and the Caribbean - and DL also has LGA plus all of the int'l operation that B6 doesn't have.
Of course B6 is trying to sign dozens of codeshare deals to try to provide the mass to compete with DL in int'l markets but DL has been able to use what B6 doesn't have to continue to grow its presence in NYC.
It isn't a stretch to believe that DL will start focusing its attention on BOS again and remain some of the markets it once flew.
DL has long demonstrated that it is capable of operating alongside low fare carriers and using its global network as an advantage.