I am not sure I can agree completely with the zero sum theory since capital keeps increasing all the time. But, you are spot on with this

Anything that has the "greater fool" theory at work is essentially a Ponzi scheme. Take the stock market. You need someone to pay a higher price than you did to make money, greater fool.

Many people have trouble getting this. Especially those who haven't traded or hadn't had the opportunity to have a business of their own. If one works for salary they are pretty much blind to what really goes on around them.