Overgrazing is a concept contrived by humans, mostly cattle ranchers. It is based on our concept of what a "healthy" grassland looks like. I even find myself using the phrase from a purely selfish perspective. In my view, an area can be overgrazed by cattle or sheep in terms of its ability to attract elk or deer or whatever game I am seeking. If, as people moved West, a group got to an area after the bison moved out and found little grass for horses, then to them it was overgrazed. Was it overgrazed to the bison? I doubt it. They ate and they moved on. They had that ability at the time. There were an estimated 50m bison on the NA plains at the turn of the 19th century. They clearly did ok for themselves.