Re: Hypthetically Speaking, What Would happen if USD collapses
Typically:
Bond holders get nuked.
Stocks rise in sync with the inflation. Of course, in real terms you will still loose money, but much less than holding cash. This is because the 'stock of companies' remains the same (whilst the money stock balloons), but looses value due to the deteriorating economy.
Commodities (including the ever so popular precious metals) rise in sync with the inflation + extra, and you actually make money. This is because the deteriorating economy reduces the productive capacity of companies, thus increasing the value of commodities now versus commodities in the future.
Regarding the ETF, if the ETF shares are legitimately "you own a piece of gold that we hold in our vault", the value of the shares does not go to zero. Instead, what you will find is that the bids for the shares simply go unanswered (no sellers), since the sellers will not accept USD for the shares. The gold will still be there. The ETF is then simply a bank, and your shares are claims for gold. They would probably then ask their share holders if they wanted to take delivery of the gold, for use in coinage or whatever.
The value of the shares will not go to zero, rather they will effectively do the opposite - they will go to infinity in terms of USD.
In reality, I think you will find that a gold ETF effectively works as a fractional reserve bank. Since shareholders rarely actually want to collect their shares of the gold, the ETF is free to issue claims well in excess of the gold they actually have.
Indeed, for all you know they maybe have 1 gold bar in their vault, just so they can laugh at it.