The only Boeings Left are the 700's. They are not due to be returned en masse any time soon. Been rumours of ONE or TWO being returned at a get out point in the leases, but just that, RUMOURS. In the mean time those 737's have to be flown and the resources for training on the Boeing have diminished as the Bus becomes the larger fleet. So if either LTN, BFS or NCL were to become airbus then those 73's would have to go somewhere which would mean either a) converting a Bus base to Boeing or b) Opening a new base and making it Boeing, both of those options are hugely expensive and h would stretch an allready over streched training department and with so many Buses on order you will never get a bus base becoming Boeing it just doesnt make sense, and as for moving the Boeings to a new base then you would have the double edged sword of setting up a new base and trying to find the resources to train a whole base worth of new Boeing pilots whilst at the same time converting the base where they have removed the 73's from to bus - just not good business sense.
There has also been the RUMOUR, sense the tone again, that LTN may get ONE or TWO buses and become a dual type base as if ONE or TWO 73's were sent back early then they would need replaced with something and Airbus is all that is available to us. Agian let me stress - RUMOUR.
Also I have, in person, heard management say that it still makes good sense to keep a toe in both manufactures pools as you never know what the future may hold.
When the 73 fleet was a mis of 300's and 700's there was much speculation as to who was next etc etc, but when the last of the 300's departed all such speculation seems to have all but seased and LTN, BFS and NCL can expect to be Boeing until a change in the position of the leases is reached or they expire - which I believe is not for some time.
ED