One of the biggest problems with retaining "Legacy" fighters such as the -15 and -16 will be parts. Once you lose a supply (or supplier), things can get lean fast. That was one of the issues facing the SR-71 back in 1990. Once a production line shuts down, it gets harder to find spares. The F-14 is a good example if this.
Another is simply age. Stress on the airframe over the years will take it's toll. Any machine with moving parts will wear down after time and you don't get the same perfromance as you had originally. Machines break. F-15 airframes are 20+ years old. Would you want to run a Formula 1 race in a car that's been on the circuit for 20 years?
Then there's competition. While there is no Cold War level of "keeping up with the Joneses" other nations are fielding new generation fighters (Typhoon, Rafale, China is working on some new programs, SU-35). While there is little chance of a conflict between the U.S. and either England, France, Russia or China, the USAF could find themselves up against a state that flies aircraft supplied by those nations (North Korea for example) Unfortunately, air combat isn't a sport and any pseudo-romantic notions of a "fair fight" belong in the movies. History shows that at the end of the day, you're either a bastard or a sucker.
And the F-4 more than fulfills the Air Force's current needs? If that were the case, the F-15 would have never been created. Besides, the Phantoms are all long gone. How many nations are still flying them? Japan? Greece? Iran? Just because the current generation of technology is good (be it aircraft, automotive, maritime, or computer), that's no reason to stop innovating. For the past 20 years we've "settled" for the Space Shuttle and it's proven to be, while remarkable, very complex, expensive and fragile. Now we realize we need a replacement, but it's probably going to be another 10 years.
Incidents flare up quickly these days, and spooling up Comanche production "if the need arrises" would take years. Tooling has been and will be lost, modified or destroyed. No, the RAH-66 is as dead as the AH-56 or the YUH-61. You're right that there was some good technology developed for the -66. Design elements will show up on other systems, but that'll be it. The Army still needs a scout/light attack helicoptor, but what shape that will take remains to be seen and should prove to be very interesting.