America has been using the F15 for 30+ years now. I don't think the phantom was a frontline fighter for more than 10 years. how long was the F4 in service or even the thundercheif or the Starfighter. With the end of the F-22, we don't even have a air superiority interceptor anymore-- almost to say that enemy fighter jets aren't even a threat.
the F16, F18 and the F15 were developed in the 70s, and they are still the front line fighter jets and multi-role attack aircraft.
I guess you could argue that the enemies of the united states are still using mig-19s or mig-21s, so why upgrade. Is that true? Russia and china have fleets of SU27 flankers, mig-29s and even su35s.
interesting how little military technology (at a macro level) has changed, and people complained that stars wars still had x-wings and tie fighters.