Ah, you're already a step further, @farmstead, hehehe, in the realm of packout. And yes, you're absolutely right: in the back the whole wall leans onto the berm of dirt, and in the front the spaces between are packed out with concrete (containing cans, bottles, and other garbage). Once it's covered, you won't even see the tires.
And yes, the cardboard will degrade eventually, but by that time the sheer mass of the wall will keep itself together. Plus, where would the dirt go, if there is more of it all around the tire? So no worries there. But thanks for your great questions. Makes me realize how much more there is that I haven't even touched upon...