AnonyMint is raising some good points.
Bitcoin is not a ponzi scheme stricto sensu because veterans aren't paid by a scammer with the subscription money of newcomers. Nevertheless, the question of bitcoin's long term sustainability remains.
I disagree with you AnonyMint, though ; I think (I might be wrong) that bitcoin-rich people will spend enough coins to allow bitcoin-poor people to trade work for coins. How do you "prove" that wrong ?