Their real aim is control--and money is one thing they can use to get that. They want to run the world--short of that, they want to run as many people's lives as they can. The money is secondary to running our lives, which is why they sacrifice making more money off educated people for the sake of preaching them away from college.
Also, if they make a "mistake", it is not really a mistake. They are aware that Armageddon is not coming at the day they say it is. What they really want is to get people to sell out and waste the next few years preaching urgently, knowing that nothing is going to happen. At which point, they will blame the flock and claim they never promised Armageddon when they really did. Frequently, it involves setting a new date or making adjustments so they can extend the window into the near future.