No, it's not just backed by mere consent. Try not paying your taxes. It's backed by the people who will make good on those promises by force.
It is true that nothing absolutely guarantees the value of the US dollar in the strongest absolute sense, but that's true of absolutely everything. There are no absolute stores of value that are guaranteed. (This bothers a lot of people, but it's true.) But there's a qualitative difference between being backed by the US government and being backed by nothing at all, a very substantial one.
I would point out that one of the usual things touted in BitCoin's favor is precisely that fiat currency can be arbitrarily manipulated by governments; this is possible because the backer of a currency can manipulate said currency, in a way that I cannot. On the other hand, I can arbitrarily issue and manipulate JerfDollars, but nobody takes them. (Not even me.) One of the things BitCoin tries to solve is precisely that people don't like what's backing the US Dollar, and they'd rather try something else. If it were indeed a backingless currency precisely like the US Dollar, the entire point of BitCoin goes up in smoke; what's the difference between two backingless currencies? Not much. In trying to claim that the two currencies are identical in this fashion, you may win the battle but you lose the war entirely.
It is true that nothing absolutely guarantees the value of the US dollar in the strongest absolute sense, but that's true of absolutely everything. There are no absolute stores of value that are guaranteed. (This bothers a lot of people, but it's true.) But there's a qualitative difference between being backed by the US government and being backed by nothing at all, a very substantial one.
I would point out that one of the usual things touted in BitCoin's favor is precisely that fiat currency can be arbitrarily manipulated by governments; this is possible because the backer of a currency can manipulate said currency, in a way that I cannot. On the other hand, I can arbitrarily issue and manipulate JerfDollars, but nobody takes them. (Not even me.) One of the things BitCoin tries to solve is precisely that people don't like what's backing the US Dollar, and they'd rather try something else. If it were indeed a backingless currency precisely like the US Dollar, the entire point of BitCoin goes up in smoke; what's the difference between two backingless currencies? Not much. In trying to claim that the two currencies are identical in this fashion, you may win the battle but you lose the war entirely.