(Before you scream "21 million": that bitcoin limit is "guaranteed" only by  fuzzy arguments about a complicated economic game, not "by math", and could be changed if the right players agreed to it.  Moreover, any kid can duplicate the amount of bitcoins in existence by creating a hard fork of the blockchain and starting to mine it on his laptotp.  Anyone who has bitcoins will gain an equal amount of those "series B" bitcoins, accessible through the same private keys, and could trade them independently of his old bitcoins by duplicating his wallet and downloading the kids's client software. Whether those "series B" bitcoins will get a significant market value is a market(ing) question, not a technical one.  And, of course, there are the altcoins.)