Ponzi/pyramid schemes require you to pay the nodes above you. At some point you cannot find new victims to add under you. On the contrary, networks like Bitcoin do not work like that. Buying Bitcoin doesn't affect the Bitcoin balances of any other node in the network.

What supports the Bitcoin price besides finding somebody else to buy your Bitcoin (greater fool)? How is much this different from a Ponzi just because it is not centrally organised?

Buying Bitcoin most certainly does effect the dollar exchange rate of Bitcoin balances and that is the only thing that really matters because there is not a Bitcoin economy independent of the fiat economy.