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In summary, technology – debit cards, online bill paying, gift cards, etc – is quickly making physical currency a niche product rather than a mainstay of the domestic American economy. The most profitable ‘Niche’ from an issuer’s perspective is the high-denomination $100 bill, and this fits neatly with the needs of many people who want a reliable and physically compact currency – especially offshore. For everyday transactions, the “Cashless economy” is clearly on its way
http://www.zerohedge.com/news/2013-10-09/rise-c-note-and-cashless-economy
Edit: he does mention bitcoin
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Then there are the more obvious sources of demand: criminal enterprise and tax evasion. Google the term “Drug bust” and you won’t likely see bitcoin thumb drives or euros stacked up next to the illegal substances. It will be stacks of $100 bills. And our Russian airport stash notwithstanding, the dollar certainly has an edge in money laundering and “Wealth preservation” strategies. Add to this the low rates of inflation in the U.S relative to many other countries, and you have a convenient (if bulky) store of value.