[engi_nerd]

The "traditional 30" is only with 4th generation fighters -- as in, we've only had one generation with that lifespan. Previous generations of jet fighters could be designed, tested, fielded, and retired within as little as 10 years as the speed of technology development increased ever faster. The reasons why 4th generation aircraft have been able to last so long are numerous. I'll take a quick stab at it.

1) There haven't been any radical innovations in jet propulsion. Sure, the latest engines from GE or P&W are more efficient than the engines of the 80s, but not dramatically so. 2) The 4th generation fighters were well designed with an eye for systems growth, precisely because the government grew tired of continually developing new fighters. Insert here whatever impact you wish to give to the "fighter mafia". These aircraft were able to take advantage of digital data buses and fly by wire technologies, which were new and allowed for years and years of expansion and growth. Point-to-point wired aircraft just get really difficult to improve past a certain point. 3) Improvements in materials and manufacturing techniques incorporated into the 4th generation gave them longer service lives.

I could keep going. But you get the idea. The 4th generation was wildly successful.

And thank you for that fas.org link, it appears to be a good gathering of facts upon my initial perusal.