You can only change the oil and tires so long before the platform wears out. The typical fighter plane in the USAF, since the beginning of the jet age, has been rendered obsolete with 5-7 years of it's initial deployment.
We have been extremely lucky with the F-15/F-16/F-22 designs because they are so good and our adversaries have not really attempted to develop a next generation threat...until now.
The F-15 is great against peer threats but has been proven to be vulnerable to the F-22 in exercises. If we don't provide capable aircraft our fighter fleet will evolve turning our planes and their pilots into a target fleet.
oh boy. What about this F35, do we have enough of them and are they as GREAT as they were supposed to be/