You keep getting downvotes because you are wrong. Alternative currencies -- digital or not -- in no way threaten fiat currencies. The reason is well-understood by economists: law-abiding citizens and businesses have frequent interactions with the government, which demands payment in a fiat currency. The vast majority of businesses that accept Bitcoin payments, even on the black market, do so only because they ultimately convert Bitcoin into fiat currency (hence the popularity of Bitcoin payment processors that do this automatically).
You are wrong about the reason. Nobody is worried that any digital currency is going to undermine fiat currencies. The concern is about enforcing the same regulations that apply to other financial services, which a lot of digital currencies have not bothered to abide by.
