I agree that at some point that may occur but I believe DL is first taking care of unfinished business with B6 and then may take on the powerful forces of AA and UA in their fortress hubs. The below proves it: I have included all destinations flown by B6 and DL including seasonal and codeshare.
B6 current BOS destinations
Domestic: ATL, AUS, BWI, BUF, CHS, CLT, ORD, CLE, DFW, DEN, DTW, FLL, RSW, HOU, JAX, LAS, LGB, LAX, MSP, BNA, MSY, JFK, LGA, EWR, MCO, PHL, PHX, PIT, RDU, RIC, SLC, SAN, SFO, SJC, SAV, SEA, SYR, TPA, IAD, DCA, PBI, OAK, PDX, SMF, SRQ
International: AUA, BGI, BDA, CUN, MBJ, NAS, SJU, STI, STT, GCM, LIR, PAP, PLS, POP, PUJ
Domestic: ATL, AUS, CVG, DTW, FLL, RSW, LAX, MIA, MSP, JFK, MCO, RDU, SLC, SFO, SEA, TPA, PBI, BUF, CMH, IND, JAX, MCI, BNA, LGA, ORF, PIT, RIC, SAV, CHS, MSY, MYR
International: CUN, DUB, AMS, CDG, PUJ, STT, PLS, MBJ, LHR, MAN
Domestic destinations flown by B6 but not by DL: BWI, CLE, CLT, DCA, DEN, DFW, EWR, HOU, IAD, LAS, LGB, OAK, PDX, PHL, PHX, SAN, SMF, SRQ, SYR, LAS
Domestic destinations flown by DL but not by B6: CMH, CVG, IND, MCI, MIA, MYR, ORF
If you exclude the fortress hubs of AA, UA and Southwest, I believe there are few destinations that DL can add which is SAN, CLE, OAK, LGB, SMF, SYR etc. I do not know how big are LGB, SMF, SYR etc. but I believe CLE and SAN are some big ones although CLE was a former UA hub. LAS was something DL tried but it stopped and I was on one of those flights I believe at some day DL may end up taking on the legacy carriers to expand.
International does not look good for B6 as barring Carribean destinations, there is none to be compared with DL. DL is almost adding all of the B6 destinations with the exceptions of BGI, BDA, SJU, STI, STT, GCM, LIR, PAP, POP.
Off this how much of these international destinations not served by DL are big is something VS4ever can exactly say in terms of pax numbers.
There are few other additions on the DL existing routes: http://www.aviationpros.com/press_relea ... stinations
See the last two para specifically so there is substance that DL is not going to give B6 easily. Exciting times nevertheless for Logan.