Quote from Dr. Zhivodka:
the Gold market is way too small to be of any consequence when fiat concurrencies fail.
This seems like the argument they made when they eliminated the last remaining ties of the USD to gold. I have never understood how having too little gold on the planet would make it a bad currency. On the contrary, the less of it there is, the fewer milligrams you need for everyday purchases. If there were 1/10 the amount of gold, we could use handy 1/10 oz coins rather than 1 oz ones. I agree if you had a substance whose abundance was, say, 1 milligram per person, it might be challenging to use it as money, but available gold is on the order of magnitude of 1 oz per person. More than sufficient to use as money worldwide.
Another example: You stated earlier that the gold market is "too small" for SWFs. How exactly does that make sense? Say it is "too small" for them to invest all their cash into gold. What would keep them from buying just enough gold to raise the price of gold by 10% every day until they decide it's time to start distributing it again? Remember, since the gold market is "too small" for them, they can keep the price as high as they want with their relatively infinite reserves.
Regarding practicality of gold in an "end of the world as we know it" scenario: When there were no motor vehicles, firearms, stock exchanges, ATMs, supermarkets, radios etc etc, a gold coin would buy a cow. After the invention of firearms, a gold coin still bought a cow. After the invention of motor vehicles, a gold coin still bought a cow.
You just have to think about the basics. Any substance that is durable, easily recognized, easily cut into smaller pieces without losing much value, scarce enough so you don't need more than you can easily carry around for everyday purchases, abundant enough so you don't have to use a microscope to be able to see the amount necessary for an every day purchase, would be a long term store of wealth as well as money at the same time. As far as I know, any such substance is called a precious metal, with gold and silver being by far the most easily recognized ones.
You are correct, wheat would work just as well, unless your goal is to save up for retirement, in which case it may be a bit bulky and difficult to keep fresh.