If you set out to be great simple numbers mean you will most likely fail. The famous "mid-life crisis" can easily result. Best to accept that you most likely won't change the world, and be satisfied with things you can reasonably achieve.
I suppose it's the defeatist attitude that bothers me.
Yes, most people will become nothing, because they were never anything to begin with. But why not embrace your potential and fight for greatness. Humility, important - just as is managing your expectations, but acting powerless is so disturbingly passive and disappointing.
You define your greatness, and you fulfill that destiny. Acting as a passive being in life is boring, and meaningless. Accepting mediocrity is just one more step towards insignificance. Grasp your purpose through greatness and find something more rewarding that "accepting you won't change the world."
The "greatest" people want to change something, want to make things different, want to leave a lasting contribution - having that desire should be universally human. Farmers and factory workers are the ones who "accept their insignificance in civilization."