Warren Buffet made enough last year to buy every single Bitcoin that has ever been mined. If he truly thought this what would stop him buying up some Bitcoin?
Except that "every single Bitcoin that has ever been mined" is not up for sale.
Only a tiny proprtion of the money supply is ever offered on an exchange order book.
Even if he did find some way to buy them all up - there's a name for that in economics, it's called demand. Such phenomenal demand for Bitcoin would translate into demand for cryptocurrencies in general if supply dried up.
In short, "buying up all the money supply of a particular coin" is not an option for someone who sees it as a threat.