-The 773 isn't exactly a best-seller either, the new ER version should bring more customers from around the world (AF, for example). AFAIK, only EK, TG, ANA, JAL, SQ, CX, and KE operate the 773 right now, mostly on short range flights.
-I think CO also operates/ordered the 739. Although it would never be purchased by charter airlines (1-class), it can still carry more pax in 2-class configuration than the 738 and perhaps carry more cargo.
-I hope someone could give some reliable info on this, but wouldn't the 753 be cheaper to buy/operate on domestic routes than similar-sized widebodies (762, 763, A310)? Also how does it compare to those a/c in terms of T/O and landing rwy length required?
-The 764 on the other hand, is indeed a failure