In case you're honestly approaching this and misunderstanding, the value of the dollar lies in that the US gov't transacts taxes in dollars. It's not just "whoever" - if BTC had a reasonably equivalently vital transaction that it could be used for (OPEC switches to BTC, EU switches to BTC) then it would be a bit more apples to apples.

> to whoever accepts Bitcoin

Double_a_92 is pointing out a distinction -- USD is pure legal tender, that is mandated by the US government to be accepted for payments of debts. Bitcoin is not.

Don't you see the difference? The value of the dollar at least partly comes from a government that has forced a demand.