So eventually when the cost of finding/creating new materials makes recycling economically feasible we "mine" old landfills and use the materials. The landfill materials become the new raw material for manufacture.
...except that they distribute the material so thinly, its unlikely it'll ever be economical to mine. Even the 'original' mines e.g. copper, iron, need very high mineral densities to make them worthwhile. Landfills are orders of magnitude less dense.
Concentrated ore is essentially solid material of a single nature. Nothing resembling that occurs in landfills. Its a big pile of 'stuff' with pellets of interesting materials every few meters. That's many orders of magnitude too sparse to be worth mechanically mining.