Not sure why I should care what Gold, or anything, will be worth in 100 years, let alone 500 years. Economies turn over money supplies on time scales of a year or two, not centuries. Individuals and business need stability over planning cycles that run from years to a few decades at most.

And it's not like precious metal based currencies were stable over long periods anyway. There is nothing in the working of economies that guarantees that precious metals will accumulate in sync with economic growth and contraction. Trying to tie one to the other is every bit as much "fiat" economics as an untethered currency, like the dollar.