Uhm no? This is illogical. You just declared money worthless. Nobody is gonna bring out the guillotines if the government demands worthless paper. They'll happily give the worthless paper to the government.

How will they acquire the worthless paper?

So this thought experiment isn't just that "everyone decides dollars are worthless" but also that the state is dissolved, along with all the property rights and the govt's monopoly on violence?

That's more like saying "Dollars are worthless in a scenario like Fallout or The Road", rather than making any relevant point about government efforts to maintain fiat currency.

In the original scenario of "people decide dollars are worthless... nothing the government can do", my point was that the government has a lot of levers to pull to backstop value, albeit at a far lower level than today's dollar.