I think gfodor is asking something more along the lines with "What can you do with gold that you cannot do with anything else you can buy?" There are valid answers to that - gold is a catalyst in several chemical reactions that won't work with any other metal, it's corrosion resistant, it's shiny, it's a very good conductor that's quite malleable. But I'm with him in surmising that if people only used gold for its metallurgical properties, absent any historical sentimental attachment, the price would drop 99.9%. Gold is actually quite abundant; 190kt have been mined so far [1], most of which is either locked up in jewelry or being hoarded as an investment, so if people ceased to believe it was valuable there'd be a very large oversupply for the few real industrial uses.
[1] https://www.gold.org/about-gold/gold-supply/gold-mining/how-...
So you'll have to convince a billion+ humans that gold is only worth it value as a metal before it loses 99.9% of it's value.
I don't know what the floor for industrial uses actually is though; if it's 10% of the current speculative price that might be germane, but if it's lower it might not.
