Arguably, the jets that entered service in the 70s represented the point where airframe performance stopped being the dominant factor, to be replaced by signatures and sensor/weapons integration.
Lots of developments in the weapons and targetting pods - plus relative lack of a peer competitor post 1989 - meant those basic airframes were "good enough". The significant (and highly expensive) exception being true LO capability.
Before those 70s frames, most types had a front-line service life of 20 years or so, less the further back in time you go. These days, the benchmark is closer to 40+ years for the basic configuration (if not batch/block).