If it is not a currency, then why does it have any value at all, other than as a ponzi scheme where new investors provide the gains for earlier investors?

When the market cap of those naive enough to buy this is saturated, then we play pop-goes-the-balloon.
I am not saying that Bitcoin doesn't show us the way towards a real decentralized currency. But Bitcoin ain't it and can't be, because there is no way to distribute the coins such that it could be, i.e. there are 7 billion people on this planet and by the time 1% of them own Bitcoin, the rest of them will be worth 1/1000000 if Bitcoin becomes the global currency. And Bitcoins are concentrated into investors hands, and investors don't spend all their networth as a currency. And if all the people don't use it as currency, then how do you invest with it and pay employees with it. Just think it out.
Distribution is the critical factor nowadays in any business. How did facebook win? Distribution!
How with the elite win when setting up the next currency regime for the world? Distribution!
Mimicking sweaty armpits Steve Ballmer, instead of "developers, developers, developers", this gorilla is chanting "distribution, distribution, distribution".
And I am not saying I don't admire the genius of what Bitcoin shows us might be possible.