(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.)
You're describing a double spend attack, right?
No, I am describing a hard fork.
It does not interfere with the original chain directly, simply creates another clone, premined and "pre-transacted" to hae the same addresses, private keys, UTXOS, and everything as the original chain. All it can do is steal value from the original bitcoins, if for some reason people prefer to spend their dollars on "series B" bitcoins rather than original ones.
a) Many, if not most, of the proposed financial functions to be performed on/through/linked to the Blockchain require a high amount of security from sybil or time warp attacks. This security is provided in the form of total hashpower that can be assumed to be honest (an assumption that is based on economical incentive, so it's a rather safe one).
Your 'Series B' blockchain is either indistinguishable from the original one, in which case it doesn't divert any value, or, if it is in
any way distinguishable, it will be ignored by the majority of miners, so it cannot fulfill the above security critical functions.
b) The other argument, "potentially unbounded no. of total coins through alts" is a better one, I personally think... I don't think this vector can be completely ruled out in the long run.
However, it ignores one aspect: the people who want to use a decentralized network as a store of value will have little interest to store value in an
alternative network that offers no improvement over the original in the following two crucial points: security, and stability of value/price.
For the 'security through hashpower' part, the original Blockchain is leading, by orders of magnitude. Stability of value obviously isn't there yet. Then again, show me an altcoin that is less volatile than Bitcoin (while at least having some of the liquidity of the total Bitcoin market) and I'll start worrying
