There's no currency backed by all world governments collectively, just currencies backed by particular world governments (usually individually, though the Euro is backed by a particular group). It's quite easy to imagine a failure of monetary policy by any one (or more relevantly, any two) of those central banks that would leave gold using industries in existence in the world, and gold in a better position than the relevant fiat currencies.

If there were a single global universal fiat currency, that might not be the case, but ethereal isn't one.