Bitcoin can not drop to zero, it is not like other altcoins that does not have value in the market. If Bitcoin is experiencing the bear market that doesn't mean the price of bitcoin will drop to zero. It is only shit coins that have the tendency to drop to zero.
The only thing that will make Bitcoin to drop to zero is when cryptocurrency is no longer being valuable again and nobody is interested to buy Bitcoin for investment and that's the time bitcoin can be not regarded as you can see, i believe that Bitcoin still have much live to go and that's it