Bitcoin can change and is volatile, its price can drop anytime, however there is no central organization that holds it which is why it would not possibly fall altogether.
Good start but wrong ending. It is exactly because of its decentralization that bitcoin can fail so quickly and drastically. There is no-one to hold it back unlike with the stock market.
But it works both ways around. If Bitcoin is centralized it can be brought down by those controlling it as fast, if not faster, should they choose so. To me, the risk of this predicament or development is a lot higher in case of heavy centralization. In essence, Bitcoin would be at the mercy of someone or someones, and this is definitely not what we all want, right?