Saying that there can only ever be 21 million Bitcoins is an empty scarcity argument, because the actual value of a Bitcoin is not yet settled by the market.
The 21 million is limited by the mining software. The USD / BTC exchange rate is not relevant to this.
Well, it is. The more value that accrues to a Bitcoin the greater the demand for subunits to fractionalise value. It's why Litecoin pisses the Bitcoiners off so much, because it is perceived to be preventing people buying satoshis, buyers instead preferring the perception of getting more for their money with Litecoin. The goal is to transition the speculation over to subunits so as to lower the price of speculating on Bitcoin and broaden the speculator base. It's why the press is inundated with fake Bitcoin stories, like the guy who lost his Bitcoin containing hard drive; 5 minutes research would show he has all the Bitcoins he ever did and it was simply sa way of getting a big news story in the press about Bitcoin. The whole thing is about manipulating people into conferring more and more value up the denominational chain.