1. Is it possible, 100 (or a certain number) billionaires buy 90% of the total circulation? If it is possible then 2. Will Bitcoin be able to maintain its decentralized position? Or will it be played by a small number of people like current fiat currencies?
Of course it's possible theorically since billionaires posess money more than the value of total bitcoin supply in the world. However you should consider few elements; first you should see how much bitcoin available in the market because not everybody willing to sell his bitcoin unless those billionaires will force holders (imaginary senario), second is that bitcoin was first made to be decentralized so of 100 persons willing to monopolize it only for themselves will lost bitcoin value after no one willing to buy or use it as it will be in the hands of limited number persons.