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.
sourceThis is pretty much all the reasons, if ever bitcoin's price drops to zero. Though it is impossible for it to happen. I just dont see another coin overtaking bitcoin nor for all the networks to suddenly go offline. If bitcoin's price fall to 5k, I'll still HODL. It always does that, then eventually rising up in a few years. However if it reaches 100 dollars, that's when i'll be worried. Maybe I'll panic and sell all my bitcoin so that I wouldnt have to waste any, just in case it reaches zero.