The idea was that he wouldn't play the other slams. Except perhaps Wimbledon. Nobody has more than one warmup there anyway, and Borg did without even that.
Really? I never got that idea. Many months ago, the quote from his camp was that he would play fewer hard court events: fewer
... not NONE. The point would be to save himself for the big stuff on HC like AO, USO and WTF.
I assumed he would skip one of IW/Miami (keeping one mostly due to points & ranking -- they aren't actually warmups for anything as they immediately precede the clay season). Then he would only play only one of Canada/Cincinnati (as warmup for USO) and then he would skip at least one of Shanghai/Paris and play WTF. So if we say: AO, Miami, Toronto, USO, Shanghai, WTF... that's only 6 hard court events per year.
Hard court tennis is a bit harder on your body than clay, but it's not a whole other sport. Tennis is still tennis, running is running, 4 hours of running back & forth, back & forth, is still tough, on any surface. The surface difference makes it maybe 20% easier at most. Either you can do it or you can't.