What is someone bought all the gold? or oil? or electricity? or corn?
That's different. We need gold, oil, electricity (highly correlated to oil, let's call it energy), corn, even gold to some extent (and we want it).
But Bitcoin has no corporal value, it has value because we believe in it and agree on its value (agree not as in fixing prices but set by the market).
One person would be pretty stupid to buy all the BTC, he/she would just be competing against themself, and everyone else would move on to create a new block-chain.
That really not likely since the next block would be worth about infinity * 50 / existing coins. It would be the most solid secure valuable block chain based currency even imaginable.