Rising interest rates, a stronger dollar and potential inflation are not the biggest threats for gold in the medium to long-term. The metal's biggest threat is Bitcoins and other digital currencies, says author and financial journalist Matthew Lynn.The drivers that pushed gold higher for over a decade are rapidly eroding. Economies and their currencies didn't collapse. Unattractively low interest rates are on the rise. And inflation never showed up to wipe out the value of people's savings.With a consumer price index under 2 percent, the lack of inflation has been punishing gold investors recently, according to Reuters. Priced under $1,300 per ounce, the metal is far from its high of $1,921.Gold rarely makes sense in a low-inflation, slow-growth economy, Reuters explained.While gold isn't a smart inflation hedge, many people have been using it that way because they think they have few alternatives, Reuters added.Lynn explains in a MarketWatch article that the rise of alternative digital currencies such as Bitcoin changes that.As a rival to paper money, gold has had the market to itself. There were essentially only two ways to stash cash — in the form of currency or in gold. Now gold will have competition, warns Lynn.He admits that Bitcoin has some road to travel to build serious credibility, but even if it fails, digital currencies of some sort are likely to succeed.Pointing to how the Internet has changed lifestyle habits from mailing letters to reading books, Lynn believes there is no reason why we should use paper money supplied by a central bank.The hoards of gold held by individuals and central banks shows there's clearly a desire for a currency alternative. But Lynn argues that digital currencies may be a better option than gold.Gold supporters often argue that gold's limited supply makes it attractive. Lynn argues that digital currencies are designed to have a permanently finite supply. You can't stumble upon a new deposit and mine more Bitcoins.In addition, many gold supporters are convinced the gold market is plagued with manipulation. It's much less likely that a digital currency would be manipulated by governments or banks. Since Bitcoins are digital, it's difficult for governments to control them or tax them and it's difficult for anybody to steal them, Lynn explains.The competition that Bitcoins creates will be good for those investors seeking currency alternatives, but not for gold or its loyalists.If gold is "not an alternative form of money, then gold is just a decorative metal, good for making necklaces and not much else. And its price will start going down," Lynn writes.Digital currencies are not an immediate threat to the gold market. But gold is a long-term investment and investors need to start thinking about the long-term impact of digital currencies, he warns.