I guess crypto will never go to zero especially the useful ones because if crypto will go to zero I think it will never been made in the first place, it is implemented because of the use of blockchain technology so I think the only crypto that will go to zero are shitcoins which are no use at all.
It will only go to zero if every person will totally abandon using it.
In any other case - crypto will has its value and somebody will try to buy/sell it (no matter how low the price is - it is a currency and it is used)