Siblings talking about the closing of the American West are correct but that's not sufficient; you have to extrapolate it out to the existential scale that Bloo

Siblings talking about the closing of the American West are correct but that's not sufficient; you have to extrapolate it out to the existential scale that Blood Meridian grapples with. He's critiquing the notion of "progress" as a whole, favoring instead a view of humanity stumbling through history and all which that entails.