"all the existing bitcoins" are not up for sale so nobody can buy it all.
lets say I have 1
BTC, at current price I am not willing to sell it. but I will sell that for 1 million dollar if that someone buying "all existing bitcoins" is willing to match me demand. and if he does then I will use it to buy cheap bitcoins at market price and have more bitcoin in my stash
![Wink](https://bitcointalk.org/Smileys/default/wink.gif)
and there are lots of people who will never sell their coins, not all of it anyways and they will hold bitcoin because bitcoin is a decentralized currency which gives us the financial freedom we want out of it without anyone being able to tell us otherwise.
when I hold 1
BTC I have a lot of power. I am holding 1 out of 21 million limited number of coins and that fact will never change, so why would I want to give that up?