Real currencies have usages, such as tax payments, that are mandated by governments. Furthermore, real investments generally aren't in the currency itself but in the capital structure of productive institutions (mostly firms and governments) that ultimately pay returns derived from the institutions' production. The currency is the unit of exchange.

Gold is an exception and over the past 200 years has produced low returns.

Many things have finite supply. That does not mean they are appropriate investments. A vast majority of them ultimately become worthless. A visible, but small minority become collectibles and museum pieces.

Even the case of collectibles or jewelry--I would recommend that they be bought for enjoyment, education, or display and not for investment. Few people buy Bitcoin to display or enjoyment, and the theme of my article is its validity as an investment.

Personally, I buy mineral specimens--for display and enjoyment and preservation of pieces of natural history. I just hope their value is high enough so that when I kick the bucket, my children find it worthwhile to find buyers who will preserve the best of them.