There are a few ways to look at this. You can say that the game today is more grueling than in the past because of the long baseline rallies or you can also look at it also as perhaps many do not want to venture to the net to put away easy shots with strong volleys. Pete Sampras was hardly known for his great stamina although he won many five set matches. He was able to serve and volley more in later years because his skills (great serve and excellent volley) allowed him too.
I understand for example that after the Laver-Connors challenge match in 1975 that Connors collapsed with cramps in the locker room. Laver said afterwards he could have played ten sets if need be.
Djokovic was super impressive with his movement and stamina today as was Wawrinka but does Wawrinka look like a Superman to you that would sweep aside all players in the past? After all you are implying that the tennis is so physical today that players of the past could never handle it. Logically Wawrinka should wear down guys like Borg and Gonzalez with his baseline play. Are you saying that?
The game is very physical today but they also have the best equipment to minimize the wear on the body. Is it tougher today? Maybe but maybe not.