Gold famously doesn't decay, but why should I expect prices to remain constant in terms of gold? If I sell you a chicken today for 80 milligrams of gold, why would you be obligated to sell me an equivalent chicken for 80 milligrams of gold in twenty years?

Gold isn't a perfect store or value, it's just a perfect store of gold.

Yea - I was just saying it's probably the closest we have ever gotten. The chicken example is interesting b/c what would you actually compare gold to? Say chickens were commonplace today and could be had for 80 milligrams of gold but then some disease wiped them out and only 10,000 could be grown and sold each year. Obviously they'd be worth more than 80 milligrams of gold - but as prices for other things perhaps come down and become more plentiful (or new products/services emerge) 80 milligrams of gold may buy something completely different at a great price.

I mostly agree with you here. Keep in mind I was preliminary responding to someone else w.r.t stores of wealth. It may even be the case that storing wealth is nonsensical as a matter of physics (either natural physics or economic physics). Perhaps what we refer to as wealth storage is really just scarcity of a physical item, whether that be a house, a collector's edition widget, a car or piece of art, or in your example, a chicken. Maybe the goal in what we would call wealth preservation is to find assets that either do not decay over time, or that decay very, very slowly over time? One thing that comes to mind then is how fiat currency plays into this decay? What would it represent?

So the question is basically "How fast does credit decay?"

Let's switch from goods to services and favors. Let's suppose you're in a real bind one day, and need some help -- painting your house, moving cross-country, who knows -- and I step up and help you as a favor, and spend a solid week of my time.

How long do I have to redeem that favor and get a week's worth of time out of you? How does the intervening time affect the answer? If I disappear for twenty years and show up on your doorstep expecting the favor to be repaid in full, is that a reasonable expectation? What if we are close friends for those twenty years, trading favors back and forth?

There aren't necessarily right or wrong answers to those questions, but that's basically the question inflation answers -- it's the price for not participating. You do a job, you get your currency, and then you can roll your currency into participating in the shared risk for maintaining our systems of production, or you can stand back, and see the credit for your past contributions decay over time. Sometimes it decays slowly, when everything is going fine. Sometimes it decays quickly; if things begin going badly and you're not invested in the outcomes improving, your credit for your past contributions declines faster.

People say gold is a good store or value because it has relatively inelastic (hard to ramp up) supply and relatively modest and stable demand (jewelry, industrial uses etc).

The cost of chicken has actually plummeted over the last century, even adjusting for inflation, because we can battery farm them now and they produce way more meat than they used to due to selective breeding etc.

Gold is still a speculative asset! You're speculating what it will be worth in the future. Its current value varies a good bit compared to other goods and dollars.