The advantage of the 739 is more seating capacity in a typical first/economy layout, as the fuselage is longer; that also, of course, means more cargo capacity. Since the 739 is larger and heavier than the 738, is will give up some range and a range of "only" about 2500 miles is inidcated for the 739. The shorter range really should not make such a difference to most airlines, as most 737NG aircraft are not assigned to long range routes anyway. Look for AS to use the 739 on high capaicty routes out of SEA, CO on the heavily travelled EWR-Florida routes and KLM on routes out of AMS to major European cities (KLM might go all 739 to LHR replacing its mix of 763 and 737 aircraft on that route, we shall see.)
Your point is interesting because many European charter airlines, who have purchased the 738 in large numbers, have not yet shown much interest in the 739; why? Your point, they already cram as many seats as possible into the 738 as allowed by the laws so the 739 brings no additional benefit to them.