I do like how cryptocurrencies are causing us to have some existential conversations about money and value and what they actually mean.

A key learning I took away from Thomas Piketty's Capital in the 21st Century was that you really shouldn't be holding on to more currency than you need to. That it's good to have useful tangible assets like real estate.

Is it accepted economic theory that the value of the dollar comes from the requirement that U.S. taxes be paid in dollars?

Does the same hold for the Euro?

Well, supply and demand is generally accepted economic theory, so yes. No one is claiming it's the sole reason for the value of the USD, but it is certainly a (huge) contributing factor.

It's at least an accepted economic theory - there is much heterodoxy in the field, and I've heard it before (and I agree with it, for what it's worth).

See modern monetary theory.