if the dollar goes to zero, that would pretty much mean no one has a job. who wants to work for $30 per hour when a loaf of bread costs $1000? owning bitcoin won't change any of that.
Of course owning bitcoin will change that.
Take any country which has recently experienced rampant hyperinflation - Zimbabwe, Venezuela, Argentina, etc. It costs billions (or more) of their local currency to buy a loaf of bread. So what do these countries do instead? They use a different currency altogether. Maybe USD, maybe EUR, maybe something else. All the people who owned this other currency prior to the hyperinflation event are in a far better position that people who are now trying to purchase this other country at exchange rates of billions to one.
If the dollar goes to zero, and it costs a million dollars to buy a single euro (substitute with any other large currency), but I still own a bitcoin worth ~26,000 euros, then I will fare significantly better than if I owned no bitcoin.
Although many were also thinking that USD will hyperinflate like the Zimbabwe Dollar, the USD might not really go that low even if it will be debased. Looking at UK's money they were not that low after the dollar displaced them, their money is still quite high even today.
Yuan may be used widely in international trade as seem to be what is happening with BRICS nation, China knows how hard it is to keep up on top which is why they are encouraging countries to just trade with currency of every country.