“Such US allies as Greece and South Korea has a 50/50 mix of US and Russian military equipment in their inventory and East European NATO allies actually has more Russian than Western stuff.”
The Russian equipment is primarily a radar. Having the two systems together F35/S-400 (or 300) would allow the users to determine the deficits of the radar and allow a laboratory to fix those deficits, thus neutralizing the advantages of this overpriced golden calf.
I suspect the shelf life of the F-35 will be short. It is too expensive for the benefits gained. It sacrifices other attributes for those stealthy attributes. Once its stealth is penetrated it will prove an easy mark for earlier, cheaper generations of fighter.
Rumors pilots are loving it.