I actually don't understand why it's significantly better for the landfill to be arranged in one way rather than another. Can you explain why it helps future generations?
If at some future time, we actually find a way to recover plastic / oils / utility from plastic that's sitting in a landfill, someone will then go back and dig up landfills to find it. If every trash bag has 10-15% plastic in it, because we don't bother to sort our stuff now, that's much less efficient than finding the part of the landfill that is 80% plastic.