IMO the really weak generation, which both the previous and next generations of players have capitalized on, are the ones born around 1975. Had these been stronger, I think neither of Sampras and Agassi would have had so many slams - and Fed a few less as well.
I agree with nearly everything you say (not completely for Lendl's competitors though),
but it's strange that you say that this generation might have prevented Fed from winning slams.
Fed won his first slam (and only one, next one in 2004) in 2003 : these players were around 28 years old, that's quite old, they should have been not only better but really great to win slams against Federer at that age.