They'll make good money at it as long as they keep recruiting. Besides the problem of duplication, it's much harder to establish a trusting relationship with someone from an ad who doesn't know you. Which, therefore, makes sponsoring, retaining, and empowering distributors much more difficult and take much longer. Getting a complete stranger to give you money and believe that you will make good on the promises you make is a long expensive process. In this day and age, with all the hype and garbage out there, especially when it comes to MLM, people are naturally guarded.
People are more skeptical today.
They are tired of getting taken advantage of, which means trust is valuable commodity. Put your energy into your warm market. You already have trust established. They know you. Remember, your cold market is someone else's warm market.