I keep hearing that there will only ever be 21 million bitcoins.
I think it's a myth for the 3 following reasons.
1) The 21 million figure could change if developer consensus did, and people downloaded the software
2) The base money supply can be easily inflated with off block systems, IOUs and CFDs for example, but there would be many more instruments to expand the money supply
3) Alt coins could become very similar to BTC, and can be easily created
This is important because if we estimate the long term % of bitcoin in world markets, you need to include these other factors, and it will also impact the price.
