The aircraft is the main issue. There are simply no more "Mid life" B738 about to buy or lease any more. The production line has effectively finished for the NG so only sold units are now being completed and mixed with Max production, much as the last classics went out the door around the first NGs. (Most of the Classics still left are of that vintage- the very last ones made). If they want more aircraft, it's got to be the Max. No choice, unless they can buy mid lifers from companies accelerating replacement of their fleets to Max.