Boeing looking at extending F-15C/D life x 2, F-15E x 4

From flightglobal:

Boeing has launched a four-year structural analysis of the US Air Force F-15 fleet, with the aim of doubling to quadrupling the service lives of the two major variants.

The USAF has also revealed new interest in critical avionics and mission system upgrades for the 40-year-old airframe, as it seeks to keep at least some of its 414 F-15C/D fighters and F-15E fighter/bombers flying for decades to come.

http://www.flightglobal.com/news/articles/us-air-force-looks-to-dramatically-extend-f-15-service-life-365200/

I wonder why this interest in extending the life of F-15 by decades… Any guesses?