Here are some scenerios that can cause the price of btc to fall drastically. (In my opinion, it is highly unlikely and the possibility very slim).
(1) No way to trade bitcoin or exchange it for goods and services.
(2) All governments of the world placing a ban on bitcoin and bitcoin related activities making it highly illegal.
(3) The entire bitcoin network and nodes going offline making it impossible to even make underground transactions in bitcoin.
(4) Another crypto overtaking bitcoin and value and more people shifting investment to this new crypto.
sourceIt is a decentralized market where the price is determined by the demand and the supply. As long as there is one person who will buy bitcoin (creating a demand), the price will never go to zero. And I am sure there is at least one single person in the earth who will still want to hold and buy bitcoin even if it crashes very badly.
Again, it is a decentralized coin and the government can't ban it. There is no central authority who government can ban. If they wish to truly ban the coin, they will have to ban each and every citizen from using it and closely monitor them.