While I do think that it must be maddening to see all that money, but not be able to touch it directly, the job isn't without perks. When you consider what being a member of the GB actually entails.
They get up and have people kiss their ass from sunrise to sunset, they just walk around barking orders. All expense trips to places all over the world where they just get up and say whatever is on their minds. Every congregation they visit they are treated like a celebrity.
They also have something that money can't buy. Power.
They intimately control millions of people's lives.
They could snap their fingers and have someone disfellowshipped.
Personally I see donations dropping as their membership numbers fall in wealthy countries. Converting their assets to liquid allows them to supplement the loss with investment money. If we get another economic crash though, they are sunk. Would need to implement new light on tithing.