One day it might make economic sense to mine old landfills for resources and then someone will do it.
Until then, burn what will burn (to get rid of disease) and pile up the rest.
Portland ME and the surrounding towns have a regional trash and recycling consortium: ecomaine. We burn our trash in a highly efficient incinerator that converts to electricity. During the ressession volume was so low that they were digging up landfilled material to burn. It served a secondary purpose of freeing up landfill space for eventual Ash disposal.
Separately, later on as part of a study of the old Ash the dug it up and sifted out the metal that the high heat incinerator had not dealt with. They were able to sell this metal and again free up more landfill space. They now sift out the metal before disposal.
As part of a prior business I would take stuff to ecomaine. I was amazed at the amount of metal people dump.y company always separated and sold our metal debris.