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.
sourceI think the first 3 will possibly render Bitcoin to worthlessness or make it price to zero but the 4th one will never make the Bitcoin price goes down to zero. The 4th one may make the price of BTC to plummet but it is not good enough to render BTC useless. There will always a supporter or buyer of BTC even though it was overtaken by other cryptocurrency.