Don't you think that, like Borg's, Fed game is too much based on his footwork to ever hope to surpass the gigantic record of Sampras ?
Borg felt he couldn't keep his quickness and fitness and decided to stop at 26. When he came back later, he was weaponless over 30.
On the contrary, Pete could rely on his serve and volley mainly, and win easy points on fast surfaces, beyond 29. Over-ten-slams winners are all serve and volleyers except Borg, who had such a concentrated career, that he consumed himself a bit, but he had no choice with his game style. I think Pete was the prototype of the "over-the-years-slam-winner", his game style being back by his consistency and racquet-do-the-talking personnality.
Roger is doing much more rallies and time could take its toll on him.