- People, as a whole, have an engrained survival instinct. Getting a large group together and getting them to agree to wipe out humanity is improbable because of this.
- The act would be the height of narcissistic behavior. The narcissistic personality disorder is a condition in which people have an excessive sense of self-importance, an extreme preoccupation with themselves, and lack of empathy for others. Going back to my first point, it is improbable that an organized group of uber-narcs would coordinate.
I do believe that mankind's hold on the planet can be shaken off, but it is more likely to be an instinction level event like an asteroid hit. There is also the interesting spiritual wrinkle of the earth "shaking off" humanity. The thought there is that humanity has nothing directly to do with its downfall. There is no evil mastermind. Instead some tipping point is reached and the earth turns against mankind. This is another novel plot, one that I believe more and find more depressing.