We are fighting a proxy war with a nuclear power at this point. I suspect this is the overriding calculation, to not provoke Russia into using nuclear weapons, with everything else merely excuses to do what we'd do anyway.

But I wonder at the training issue. We spend $millions per pilot to train them to use an F-16, and it'd take at least a year. We don't want poorly trained pilots getting shot down immediately with Russians picking through the wreckage recovering tech to reverse engineer.