Actually about 20% of USD is tied up in the black market and about the same percentage of BTC are used for black market transactions too.
Even if Satoshi Nakamoto does sell his/her/their BTC, it’s not going to change much for Bitcoin itself, aside from causing a huge sell-off. It’s no different than somebody liquidating a huge stake in a commodity market. Of course, such liquidation is quite risky for the person attempting to sell: selling a lot of BTC at once will cause the person to lose a lot of money because the market is simply not liquid enough.
The identity of Satoshi Nakamoto is about as important as the identity of the inventor of the Transmission Control Protocol (TCP), I bet you 99.9999% of the people that use the Internet have never heard of Vint Cerf nor do they really care who he is. Satoshi Nakamoto has no bearing on Bitcoin, how people use it, who uses it, why they use it, how much do they decide to trade it for, or anything of the sort. So why does the identity matter? All he/she/they can do is sell their current BTC, but even that’s going to be tough without a major loss.
The US government doesn’t actually back the USD, it merely issues it and requires its use. The US can’t guarantee the value of a dollar, all it can do is provide “stability” in order to give confidence to the people that hold the USD. Bitcoin just works differently: it’s issued without an authority, its use is not mandated by anybody and confidence relies entirely on the infrastructure of businesses surrounding Bitcoin. The volatility exists because it’s a new protocol, there are regulatory questions that still remain unanswered and the economy is not quite mature enough to provide a more stable exchange rate. If you’re concerned about the price you can mitigate that risk by going through a payment processor like BitPay, but the more people use it the more stable the price will become.