That said, even before Bitcoin arose, most real-world (fiat) currencies were handled primarily through electronic transfers. There, the double-spend and scarcity-preservation was largely maintained through a combination of accounting convention, interbank agreements, audits, regulations, and occasional criminal prosecution (for things like counterfeiting). So "scarce digital assets" in the form of entries on the Fed's (or the ECB's) balance sheet existed before and continue to exist after the emergence of blockchain-backed cryptocurrencies.
Gold-backed ETFs represent another form of "scarce digital asset", but more of a hybrid asset since they only arose and kept their value stable after creation/redemption mechanisms were formalized, and continue to maintain counterparty trust through auditing. But they do provide a liquid means of owning electronic gold; gold being the traditional scarcity-based asset.