Are cryptocurrencies a threat to the dollar?
Personally, I do not think so?
Nope, or at least not yet. Government-issued currencies are still needed to pay in the country's stores, to pay taxes and so on. Unless a government ever switches from its currency to Bitcoin, they can work very well in parallel. And this will not happen in the near future.
But Dollar is not used only for payments in US. And here the interesting part comes. Many things' prices are in Dollars over the world, oil is one of the most known. If such things get traded against something better than US dollar (yes, I mean Bitcoin), then Dollar will have a problem: it may get downgraded from the list of global (hard) currencies. But it's again, quite unlikely to happen in the near future. And it will not be only the Dollar in this situation - Euro and maybe Yen will be in the same situation.
TL;DR; While there are directions to think of where Bitcoin could become a threat for US Dollar, none of those have a chance to happen in the near future.