"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
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?