Now my question is: 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?
They can if they want to gatekeep and keep the 90% of the total supply in the economy right now. Let's just deduct the lost Bitcoins forever and those that haven't been mined yet.
So, for them to do that. They'll have to contact all of exchanges and the miners that they'll have an organized buying that they would do.
The winners here are not just them but also us who have been holding for so long. If we start seeing them seriously buying, are you going to sell? probably not. So in theory, they will be able to but it also depends on the supply in the economy and if most holders are going to sell if the demand keeps up and reflects to the price.