Or maybe the conditions (surfaces, balls), tech, training methods, nutrition etc. differ from era to era and players happen to simply be a product of their era?
I know people will once again bring improved records in swimming, racing etc. but tennis is far more nuanced than those sports, bigger, stronger and faster athlete doesn't always win.
Things like feel for the ball, anticipation, court sense etc. play a big role as well, tennis court isn't exactly big and thus isn't that hard to cover (making anticipation for example almost as much as a factor as raw footspeed).
Did you watch the last match of 2012 season? Between Stepanek and Almagro who was the younger player who hit with more power? And yet he lost.
Can't disagree with that! Totally logical. (I'm beginning to sound like Mr. Spock. Wish I was as smart as Spock. )