https://www.youtube.com/watch?v=SVp-zIONsrsif someone started to buy up all the btc in the world, if they did it fast enough, could create their own bubble, a huge increase in price followed by an equally huge crash, and then months if not years of stabilization.
if they did it slow, they would gradually increase the price, which could in theory bring in more vendors wishing to sell their products for this new and lucrative commodity/currency. however, most likely the bitcoin economy would not be able to support that kind of growth, and you would get another collapse.
i think ideally, if you wanted a billionaire to voluntarily buy enough btc to make most current holders rich, it would have to be incredibly slow, or wait until there is much much much more liquidity.