One of the common tropes in arguing over virtually any subject is the notion that if we don't have an alternative theory of our own to the one proposed, we have no grounds to criticize a particular theory. So for example if we don't have a naturalist alternative to the current prevailing Darwinian theory of origins (whatever it is today), that means we are in no position to criticize that theory. And if we don't have an alternative comprehensive theory of how the legitimacy of government authority develops, that means we have to accept that it develops under some theory in which it derives from consent.
But this is just obviously not true. We know that the legitimacy of government authority does not derive from consent, even though we may not have some comprehensive theory of how concrete instances of legitimate authority develop. We know this because the good is something to which we are obligated to conform our wills, and not vice versa. What is good does not derive from what we will.
Does this explain how legitimate government authority develops in the concrete? No, no more than a criticism of the weakness of random mutation and natural selection explains how a world filled with nothing but prokaryotes became a world filled with mammals.
But it isn't necessary to present some alternative theory in order to know that a particular theory or class of theories must be wrong. Often the best theory we have is no theory at all.