As a thought experiment, what happens if one person has collected - say - 80% of the money as some sort of dragon of Norse myth, and the rest of the economy decides "we don't want to be subject to that guy's whims - we'll make a new currency". Unlike the dragon's hoard of gold Bitcoin has no value as raw material, unlike fiat it has no backing from the government and courts, and unlike physical money has no numismatic value (the blockchain is public anyway) - so there's nothing to stop the rest of the market to decide that they don't want anything more to do with the Bitcoin currency.
If the dragon lost the purchasing power of his gold, he could at least swim around in it, and maybe earn some new money for food by charging admissions.
Nice thought. Just a little modification - how about of 50% of the money? That is what one party has in Syria - and that is each month! In fact, it is the definition of hyperinflation *).
It is probably not easy to get 80% of all the bitcoins, and it will be even less easy with time.
*) It is defined by 50% price increase per month really, but a definition of money supply inflation of 100% per month could also work.