Here's Borg and Lendl in a 921 shot rally at the 1981 Masters. Judge the height of the ball over the net for yourself. Remember that 6 feet over the net means that one the players could stand on the net cord and the ball would graze the top of their heads.
FWIW, I'd estimate most of their shots are around 3 feet over the net, or about 6 feet off the ground.
Here's Jimbo and John from the 1984 USO semi. Again, I'm seeing a couple of feet max. net clearance on balls that they're hitting offensively. Neither guy is Mr. topspin. There are defensive shots that are higher. Overall seems lower than Borg and Lendl.
Fast forward three decades to Fed and Djokovic at the USO semi. With the pace those two were hitting with there's no way the ball is more a foot or so over the net on most shots. I am still in awe of the pace and consistency that they generated.