Good point The_Cutest_of_Borg
I simply don't know the answer, and look forward to seeing how DJ are going to handle it (definitely more uncharted airline territory – this will write a new chapter in the book).
The competing A320 series offers a stretch to 220 seats, and this has been one of the reasons behind its adoption by some airlines. But if DJ take it on, or move to another 220-270 seater, there goes a couple of the key LCC attributes. It's a tough decision. Maybe they'll do nothing for a while (apart from adding yet more flights).
Godfrey himself gave only limited insight in his AA interview with Jim Thorn – the extended interview is on the 'net:
http://www.ausaviation.com.au/Aboutu...grounder1.html
We’ve looked at the -900 series and it just doesn’t stack up. Between our 144 seater and our 180 seater we have a marginal improvement in cost for ASKs (available seat kilometers) going to the bigger aeroplane. The problem with the -900 is it’s not like the A321, the -900 is only another 19-20 seats, for those extra 19-20 seats you have to put on an extra flight attendant. You don’t get the ratio benefits we get with our existing aircraft. It’s a bigger aeroplane again and I can tell you the cost savings are not there for us to be interested enough for it to be in our fleet.
What we are doing with KSA is that we are swapping our -700s with -800s, particularly with Sydney/Melbourne and Sydney/Brisbane where we find that were doing very well, instead of having the 144 seater at peaks we go to 180 seater and that’s money for jam. Ideally if the -900 was a 220 seater, like the A321, I’d be very much more interested in it.
That doesn't really answer your question of what happens as they grow beyond the 737-800. Boeing reckon that the 737-900 is the ultimate stretch for the 737 "The 189 passengers permitted today is the maximum number able to pass quickly enough through the doors and evacuate the relatively narrow emergency exits over the wings." Those not in the know reckon that the –700/-800/-900 was already a stretch too far for the original 737 certification. After the questionable USAF 767 tanker exercise, IMHO Boeing won't get the nod to stretch the 737 any further. So there is no relief in sight from Boeing.