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.
what you say is true, because bitcoin if the price goes down it is a good opportunity to buy, unlike altcoins if for example it goes down people will hesitate to buy even the altcoin will be worth zero. so in conclusion bitcoin will not be worth zero.