but amway can survive for decades

Also Amway makes it a must for its members to try out the product first before selling it to others, therefore even if the member does not manage to sell a lot, Amway still gets lots of sales from theri own members.

Another is the mentality of people in these MLM, the MLM convinced the members that they are smart and making the right choice for joining them, so the members will defend the MLM and their products to death because they think they can't possibly be so stupid to get cheated and conned into something like this.