Earlier quoted context omitted.
You could pay taxes using the German Papiermark or Zimbabwean dollar, and look at what happened to them... Intrinsic value is not determined by the ability to pay taxes. Fiat money has zero intrinsic value (well, beside the value of the paper and metal bills and coins are made of, but this value is negligible - spare the US penny coin).
Obviously the importance of paying taxes directly depends on the strength and stability of the government collecting them. I am personally sure that 1) the US govt will be here tomorrow to collect and 2) they have enough guns to do so.
What you are trying to say, I believe, is that fiat currencies will likely have some value in the future because they are in demand (notably from taxes). That is true, but this is no different from Bitcoin: it has value because it is in demand. I would argue that the potential demand for Bitcoin is higher than the potential demand for USD, if only because there are billion of people across the world from countries with terrible fiscal policies for whom Bitcoin is clearly superior to their local inflation-ridden currencies. Compare this number to the merely 200 million tax payers in the USA.