I guess I just think about UPS, if you're going to fly an airplane 4 hours a day/night then why buy a new $70M to do the job when you can buy a 727? For a multitude of reasons it'll save money despite not being as fuel efficient. Now, you're going to fly an airplane around the world 21 hours a day to your most important destinations, what do you get? A 744F, new, under warranty, more fuel efficient, less likely to have problems, etc.

So, do you want to put a DC9 up to 15 hours or more a day? I mean, how many cycles would that be on the frame? Where is the break-even point between over/under utilization and is it a cycle or hour issue?
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Of course, weren't we selling 2 ERs because they'd hitting their limit on cycles and it was smarter to sell them then just run them into the desert? Is it true too many ATL-Florida flights ruined a plane that could have lasted longer if it had just been in constant motion over the ocean?