It really depends how willing we are to reuse the space. If we regard a graveyard as a place where we bury a body in a designated spot and that spot is then permanently sacred, sooner or later the entire earth will be a giant graveyard because people continue to die and in that model sooner or later we have to run out of space.
The eternal problem is figuring how soon it's appropriate to reuse a graveyard to bury more people, and I guess one problem is that it's hard to come up with a one-size-fits-all approach without making it an inordinately long time. I've known people who still regularly tend the graves of loved ones who died decades previously, and others who all but stop visiting a grave within a couple of years. Then you have the question of how long to preserve a grave if people who visit live abroad - I have a family member buried abroad and typically visit the grave when I'm in the right location, but typically only get there every few years. Should that grave be preserved because I'll visit it even if only very sporadically, or reused within a decade or so of being first used?