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 just went through the list and I still can't find a credible reason for bitcoin to zero out, there's no stopping this evolution no one not even the government can be able to stop it, that's why even in countries where bitcoin it is prohibited some citizens in that country still make use of bitcoin through bitcoin can never be worthless no matter what you guys think might happen in the future.
(4) Another crypto overtaking bitcoin and value and more people shifting investment to this new crypto.
I thought we are done with this debate of another cryptocurrency taking bitcoin's place, it's never going to happen even if the supply of the crypto is lesser than bitcoin's 21 million.