The point is not that bitcoin will or will not fall to zero, but that the blockchain will continue to work well anyway. Although zero is impossible because the measure of value will still be there, bitcoin can still be used to buy or sell goods. If the dollar becomes weak, bitcoin will be measured in another currency.
In terms of value or price, Bitcoin can actually be measured by any fiat currency as long as the currency also has value so this clearly doesn't depend on the dollar currency. This is what makes Bitcoin not easily become zero again as long as most people can still measure its price and value with any currency, because it is always based on the level of interest that exists for Bitcoin which also causes the blockchain to continue to work when there is a transaction