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).
The market need it for volatility. Without volatility, you can not trade bitcoin.
(1) No way to trade bitcoin or exchange it for goods and services.
You can not trade bitcoin on centralized exchanges but there will be many other ways to trade bitcoin. On peer to peer marketplaces, on decentralized exchanges. Another way is direct exchange with bitcoiners around you. Everyone can make direct deals with each other and finish their trades.
Escrow services will be mandatory for your safety.
(2) All governments of the world placing a ban on bitcoin and bitcoin related activities making it highly illegal.
How many years since the birthday of bitcoin? 12 years and governments won't ban bitcoin or cryptocurrency as we have seen. They only have more rules, regulations on this market. That's all they will do.
(3) The entire bitcoin network and nodes going offline making it impossible to even make underground transactions in bitcoin.
If you have electricity, power, infrastructures, mining rigs (ASICs), and skills to set up your rigs, connect to bitcoin nodes, pools, you can mine. The network will keep moving forwards.
Apocalypse will destroy all things on this planet and bitcoin network will be killed only by this way.
(4) Another crypto overtaking bitcoin and value and more people shifting investment to this new crypto.
Altcoins? They are shit coins and they can not kill bitcoin. They can only kill themselves, make suicide and end their lifespans.