If you don't mind, let me try doing it with a question...
Imagine your ideal world where there are no banks and we're on the gold standard. Suppose that you're a wealthy apple grower, while I am a struggling manufacturer of widgets. If I come to you to borrow some money for 3 years to build a bigger widget-making machine, will you lend me the money interest-free?
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My ideal world includes banks and an elastic currency. You're making assumptions and they're incorrect.
As for the question - no, I wouldn't lend you the money. The gold standard assumes a deflationary price level, and holding money is superior to lending it out, assuming the interest cost is zero. Interest has to be paid to compensate for loan risk.
In todays world, banks loan out money they create as digital ledger entries, and the value from that loan is derived from all the holders of dollars, who are robbed an incremental amount, when that loan is spent. The owners of the currency are the people, and the people have to be compensated for the principle they forgo when a loan is made (decline in a dollar holders purchasing power via an increase in the money supply when a loan is made).
Your example proves my point. The loaner of the money has to be compensated for the loan (the people).
P.S: Your definition of seigniorage is incorrect
No, it appears your definition is incorrect.
The federal government realizes a financial gain when it issues notes or coins because both forms of currency usually cost less to produce than their face value. This gain, which is known as âseigniorage,â equals the difference between the face value of currency and its costs of production.
http://www.gao.gov/modules/ereport/..._savings/General_government/42._U.S._Currency
The point is that like Governments, commercial banks also create money and their production costs are near zero. From which they also derive a profit from the creation of that money. Not from spending it like the Government, but from loaning out, to earn interest.
It's the power to CREATE MONEY that is the essence of senioriage, and commercial banks have exactly that. You are splitting hairs and being difficult. Even the Dallas Fed implied commercial banks have the power of senioriage (which all banks clearly do), however, their focus in a research paper was on the Central Bank or Government benefit. Not the private benefit commercial banks receive.