Apart from natural disasters (some of which are partially man-triggered) I think the biggest danger is greed.
It drives out worse of the humanity, and causes so many issues and problems...
How do natural disasters became Earth's greatest threat when we, humans make those natural disasters more dangerous and more stronger. We cut trees, we create pollution from air to noise, yes you have a point that greed is a bigger danger but the biggest of them all are us, Humans. We are the threats of our planet, and we also know that we are the only one who can save it,