Gotta disagree with you here.

The early 5500 and pre-ship 622 series 757's will be 30 years old in 2016. I don't believe these aircraft can be wingletted due to a different wing structure than later aircraft. Including those aircraft, there will be around 75 757's over 25 years old that year. The earliest A320's (about 18 a/c) will be over 25 years old. 89 of the MD-88 fleet will be 25 years or older, not to mention the age of the 34 DC-9-50's we still operate. 10 of the 16 744's will be over 25 years old, as will a number of 767 domestic and 767ER. While the MD-88's have fewer structural concerns, that's a lot of older airplanes. There's just over 100 MD-90's in the world, and we already own 49 of them according to CPO numbers. FedEx, UPS, and DHL are scarfing up all the low cycle 757's they can acquire on the used market and turning them into package freighters, making the used aircraft market pretty competitive.

I don't see how you make it to 2020 with just MD-90's. Pushing it out that far would make a huge CapEx hurdle during that decade as you tried to replace substantially all of our fleet over 10 years.