I don't see why it is inherently unsustainable. There is nothing in the very nature of disposability that precludes a robust recycling path to close the matter

I don't see why it is inherently unsustainable. There is nothing in the very nature of disposability that precludes a robust recycling path to close the matter loop. And nothing that precludes including the cost of that recycling in the price of the items that will eventually be recycled and/or the next things made with the material. Trees dispose of their leaves, those rot and feed a chain of creatures and the material is recycled to the trees. This is something of an existence proof that such a system is possible. I would agree we currently do a terrible job of approximating any such ideal.