$10,000 in The S and P 500 with dividends reinvested since September 71 blows $10,000 in gold in September 71 out of the water. It's not even close. 3.2 million vs 950 K.
To your point however - the real theft here is in the unit of account.
Imagine if you will, that those impressive investment returns you cited in the stock market were not denominated in an ever depreciating currency, but were denominated in gold.
See how that works?