Total Ownership Cost is how you determine the lowest cost.
No doubt the current Boeing Airframe is built to last a long time. I understand Boeing Engineers wear both suspenders and belts on their pants, so if one fails they won't loose their pants, and that is how they build their products.
Steelcase makes great office furniture. In the past their desks have been made to last forever. Their current desks are still very well made, but not as good as rheir past models. The reason being, facility managers want to change furniture often to keep office looking morden. Though the new desks cost less to purchase, the per year cost is probably about the same since they are planiing to change the desks more frequently.
With changes in composite materials, wing design, and engine improvements. Airline management may not want an airframe to last 30 years anymore, maybe only 20 years. So they can get rid of the plane and get a more advance design sooner. SO they don't want to pay for the extra ten years of life at purchase time.
Years ago when the 737-300,400s came out airline did not rush out and re-engine the -200 fleet. Why, not cost effective. About the only cost effective change to older planes have been the blened wingletts, Why, raising fuel costs.
In the past at Beancounter Air (NW) braged how it was very cost effective to keep the DC-9s since they were paid for (very low cost of total ownership.) With the current cost of fuel, NW can't wait to dump their forty year old Mad Dogs. And NW would probably would have been better off if they dumped these years ago for a more fuel effecient plane like the Aibus 320 that they have in their fleet.
Times change and along with that are the economics. People here assume management is planning to keep the newer plane on the property as long a the current 737s, 757s. If the initial cost is lower (like the newer desks) The plane could be to up-date more often, and take advantage of the lower initial coast along the way. And then the total ownership cost would contine with a newer more advance plane.