In aviation circles we have an adage:
"The most useless things you have is the sky above and fuel behind you", meaning altitude you can climb to and fuel you left on the ground.
Military add's one more, the ammo you did not bring.
So, I agree with the idea that its what you bring on an airplane is more important than speed, stealth, maneuver, and what not. Also goes with calling all vehicles today as "platforms".
In that regard, perhaps the MiG-21 is average? Dunno.
As for longevity, exactly as you eluded to, very low cost of parts and overall maintenance. It is a small plane too, like the F-5, which also keeps costs low.
While my statements are relatively salt free, should be sweeter than most opinions out there.