"Usury" is still a word in use but now, for legal purposes, it means "interest that a lender charges a borrower at a rate above the lawful ceiling on such charges" rather than just any interest at all.
traditional usage of usury was the same, any interest charges at a rate above the lawful ceiling, just where the ceiling was 0% for christians and in a like manner for moslems.
As you point out, the difference between money and the tooth fairy is between their characteristics and rules. Economic theory then focuses on money, the economy, and their characteristics and rules to develop insights into patterns, behavior, and previously unseen characteristics inherent to economic theory.
Anyone is free to create a theory of tooth fairies and attempt to find similar relevance.
By that reasoning, there's no such thing as "chess theory" either.
> with the end of scarcity
Scarcity ended? Nobody told me!