What I was trying to get across was as the design layout remains roughly the same, two engines, two vertical tail planes, conventional wing layout , same aircraft layout, that way each model no matter the differences learns from the predecessor as to how to improve the design, it evolves.
Chopping back and forth, conventional, swing wing, delta, conventional, that evolution doesn't tend to happen as they in effect start from scratch each time, and is a more expensive option because of that fact.
Nutloose I think general configurations are copied, the trick is in the detail. I think the Mig25 was from the sixties (without implying the F15 was a copy). The russian R&D (Tsagi) did dozens of models and configurations together with a string of design bureaus to come up with the most suitable designs to meet requirements.
What amaes me on the russian fighters is the way they put full thrust on the engines without stalling them at all speeds and angles. The Mig29 could even shut off its intakes for bad /soft runways..
MiG-29 FULCRUM Wallpaper free wallpaper download no19232
The big sukhoi fighters like the Su-35 have enormous radars, range and weapon load too. And are successfully exported everywhere. Scaring e.g. the Japanese and Koreans..
The Chinese are testing their copies on aircraft carriers, earlier then anyone had hoped. The USN is looking brave with their Super Hornets and JSF but doesn't like it either. Let alone the big Chinese stealth fighters.