It wont go to Zero at all, because there is no reason that should make it go that down. The other cryptos will also not be worthless because they will continue to grow rather the going down. Your "if" is full of negative energy for us the community of the crypto currencies. The ones who are investing in the cryptos will continue to invest in them for long time. There can be some possibilities of it going down, and that will only happen if there whole countries start to ban the keeping and usage of Bitcoin. However, it sin't gonna happen any soon for now. Even, if the whole world started creating their own crypto currencies, they still wont be able to stop Bitcoin's growth.