The answer is simple - cost, keeping an out of production fighter in service and operational becomes more expensive with each passing year. Also, the more types you have in service the less efficient you are.. Think about, training, logistics and knowledge required to support legacy programs. Many don't realize that a single F-35 has more computing power than the PAF's entire fleet of F-16's. There is no way we can get that kind of performance form legacy fighters, there is just no room for it.
Also, the F-35 is not a good fit for India. Just in case anyone thinks I support the authors point of view.
Well, my point is why stop the production of 4th gen fighters at all? Why not keep a production line for one of the F-teens open? The most advanced F-15 (is it the silent eagle?) with stealth features and RAM coating and an AESA radar will give ANY air force in the world a tough time. Or the F-16 IN that was offered to India in the M-MRCA competition - I am sure that the production and operational cost of that during it's lifetime would be way less than that of a stealth aircraft like the F-35. Shouldn't the US keep a production line open for one of these, to reduce costs over the next 25 years?
I understand that an F-35's avionics and mission computer are several times more powerful than an F-16's. But how much computing power do you need, to drop a few bombs on a Talibani sniper, or to send a missile into a cave complex in Tora Bora? The point I am trying to clarify is this - why use the operaational life of a very expensive F-35 to do such menial tasks, instead of using a humble F-16? I'm not arguing against getting F-35s, but shouldn't US also maintain a fleet of less expensive aircrafts? Use the stealth fighters to take on China or Russia, should the eventuality arise; but while that is a remote possibility, the possibility of USAF having to fight wars against foes like Iraq or Taliban or rag tag Libyans is much more.
Take the present situation, for instance. The USAF could not aquire as many F-22s as they wanted, due to cost reasons, and had to cap it at 189. But many of its squadrons are facing issues of ageing aircrafts, due to the delays in the F-35 program. Why not equip those squadrons with brand new F-15s or F-16s, with the best electronic wizardry? They can still overwhelm the air force of ANY other country on earth.
Of all the combat missions flown by the USAF in the past ten years, how many of them would have been better served by an F-35, instead of an F-teen? In future too, I don't think that will change. The majority of missions will be against insurgents and rag tag militias. And using an F-35 with it's highly advanced avionics and stealth coating would be tantamount to using a lightning bolt to kill a bug.