There are many possibilities for bitcoin to go to zero, IF:
1. There is a world war that attacks the internet connection and everything, so that makes people can not access crypto aka bitcoin.
A real world war will most likely mean a nuclear winter. You won't have to worry about Bitcoin but how many years you have before you get cancer from radiation.
2. The government blocks all access to the internet, so people can not use the internet as usual.
A government? Which one? If the US gov does it people will flood the streets. It's possible in China or North Korea AFAIK there's no world government yet.
Also, if you can't use the Internet you can't sell your Bitcoins and make the price go to 0.
3. There is another technology that will be more potential than bitcoin.
There are already altcoins that are faster and more anonymous, but it's Bitcoin that is worth 15k USD.
4. All of the whales sold all of their bitcoin at once in all markets.
No, this would not cause the price to go to 0. All that it would do is create a temporary flash crash.
The concept of whales owning most bitcoins in the world is greatly exaggerated. There are billionaires who own Bitcoin but none of them would want to tell others that they're planning to sell because the first gets the best price and the last seller loses the most.
5. etc.