Take a look at the 1950s...
https://en.wikipedia.org/wiki/Category:United_States_fighter_aircraft_1950–1959
From the 60s and later, the US produced less and less designs. Why?
Because if you look at the 50s designs and study their deployments, you will see that each design was essentially an execution of a narrow field of aviation. Take the F-104, for example, it was designed as a pure interceptor -- get there and do it fast.
Later decades have fewer designs but each design is more complex and expensive, and more versatile and lethal. The progress was and still is inevitable.
The F-22 and F-35 will defeat everything the 50s/60s/70s produced, and will defeat most of what the 80s/90s produced. Right now, the rest of the world is in the 80s and 90s.