I'm pretty new here so correct me if I'm wrong, but it seems to me that this could go on a loooooooong time.
What am I missing here?
There are a fairly large supply of idiots in the world, but the number of them that are capable of transferring BitCoins is not so large. As soon as you run out of idiots with BitCoins, the game will end.
For every N BitCoins deposited, the pyramid must pay out M BitCoins, where M is bigger than N. So with every deposit, the scheme falls further and further into debt. The longer the game runs and the more deposits it gets, the greater the total loss will be.