Nothing, but if I earn $1 on January 1 and anticipate 30% tax on that due April 15, saving $0.30 guarantees that I can pay that debt. If I earn 1 mBTC on Januar

Nothing, but if I earn $1 on January 1 and anticipate 30% tax on that due April 15, saving $0.30 guarantees that I can pay that debt. If I earn 1 mBTC on January 1 and save 0.3 mBTC, then that might be worth much more or less than my tax liability come April 15. The tax argument for the value of USD is because people have future debts denominated in USD (taxes), and they become aware of the amount of those debts in USD before those debts are due. If those people want an asset perfectly matched to that future liability, then they want USD. The demand is from that saving period, not the actual payment (which you correctly note is brief, just however long the transaction would take). Most discussions either express this distinction unclearly or miss it altogether, including the linked article.