So no, from a rational point of view, gold doesn't actually make the most sense unless it can be easily converted, or you're living in a pre-industrial revolution civilization.
In most countries gold can be easily converted to local fiat and that makes it a sound alternative currency.
We cannot prescribe a universal rule for gold's attractiveness as a diversification tool because different economies have different relative results.
If you live in the US of A, you know gold was at 1800 and is now at 1300. You are losing, in terms of your local currency, if you bought the top / ATH.
If you live elsewhere, things might be different and gold may be at an ATH right now due to currency devaluation. The gold price chart in AUD, NOK, CAD, INR, RUB, ARS, BRL etc are quite different than the USD chart... Thus gold is waaaay more attractive in certain markets compared to local fiat. Holding local fiat is what doesn't make sense under these circumstances.