> When a pile of "capital" is destroyed (i.e. a bushel of wheat spoils, or a factory burns to the ground, or the lyrics to Bohemian Rhapsody are somehow lost forever)…
Well, at least one of these examples is actually capital (the factory). Of the remainder, one is just plain property, not a capital good, and the other is information. Capital is property which serves as a means of production. Sometimes the term is used loosely to include highly marketable goods (money) which can stand in for the means of production (e.g. "raising capital"), or even more loosely as in "human capital" (referring to the capacity for labor as a means of production), but it does not include commodities consumed in the production process, such as the wheat; nor information, which is non-rivalrous and thus not property, much less capital.
With that said, you're basically correct about the difference between currency other forms of property—goods with utility beyond their use in trade. For the most part there is no advantage to society in having a larger or smaller amount of currency (in aggregate). For economic calculation it's best if the supply just remains constant; any variation will temporarily affect the allocation of other resources, but eventually the value of the currency will adjust so that the total purchasing power of the currency equals all the goods available for purchase. It can cause problems if the supply changes too rapidly, making prices unstable, or if so much currency is lost that it starts to impact divisibility. For example, if bitcoins grew in value to the point where one satoshi was worth a month of labor we would need to modify the protocol to add more significant digits, or introduce a parallel system for day-to-day transactions, much like silver vs. gold.