One might counter-argue that it is actually "evolutionary" upgrades and successive designs since the late 60s/early 70s that has contributed to some degree to the length of time and cost to develop the F22/Typhoon/F35 etc.
The 50s to mid-60s could probably be termed revolutionary when you look at the number and range of different airframe / wing / engine technologies that were designed and built over a relatively short period. F106 through F111 to F15 was what, 20 years tops? Last time I looked it's 20+ years since the Strike Eagle entered service.
What changed? McNamara - definitely. Oil crash in 73? Fall of the Wall and disappearance of a near-term threat?
Probably all of the above. But the intriguing thing is "cost"which is what drives the "unaffordable" perception. Every single western aircraft since the F18 has had a contract requirement to try and drive out maintenance manhours. The use of sims has resulted in a dramatic reduction in flying hours, so you'd think that somewhere these savings would accrue. It ain't the military industrial complex keeping it either, because there's a whole lot less of it in employment terms - something else not often recognised.
Cherchez la bunce!