I'm not convinced it's tax collection specifically that gives value to the dollar. If the US government collected some taxes in bitcoin it would still have to pay its obligations on existing dollar-denominated bonds. So it would sell the bitcoin in exchange for dollars, thus creating demand for dollars.

I think that if people increasingly wanted to use cryptocurrency over dollars, the Fed would nevertheless be able to maintain the value of the dollar even as they got less common. Eventually the currency would disappear when the last dollar was traded to the Fed to pay off the last dollar-denominated bond, and I think dollars could maintain their value until that happened, even as the government started collecting taxes in other currencies.