Sampras also played in an era with the most polarization of conditions vs. the most varying display of specialists the game has ever seen.
Its not that difficult to dominate under homogenized conditions. (As the Big 4 have proved). Playing under slow medium conditions today would be like giving Sampras year round fast conditions in the 90s.
You can't compare the eras in this regard. Hell Nadal may not have a winning percentage over 30 percent if he played in the 90s. Nole as well. Fed wouldn't have fun playing against Specialists all year either on every surface. You have to plan for a gazillion different styles of players under various conditions
I could dominate Call of Duty Black Ops 1 year after year and destroy 95-98 percent of the competition on earth with a CONSISTENT 25 or 30-5 K/D ratio. Why? Because its the only multiplayer I like an play and and have found I can play the same way all the time and dominate playing the same way over and over with the SAME GUN the Famas and Same load out. (Nadal, Nole, Federer baseline.. No need for all court play, and different tactics and schemes), After 5 years of playing the game and getting good doing the exact same thing (Same Create a class, Same type of game style, I RARELY have ran into anyone who could up me in that game. . My domination wouldn't be as good if I had to play all 3 COD Black Ops games along with Advanced Warfare and The Older Modern Warfare games (Mens game in the 1990s) . Its too much varying Gameplay all the time to see year round domination