we will get back to the stone age between those two things.
(Larry Niven addresses this somewhat in his novel The Mote In God's Eye. I can't really elaborate without spoilers. :P)
Speaking of cyclic history, I also liked "The Peddler's Apprentice".
Plus, you're not going to find coal or oil at a landfill, except in the form of plastic.
Energy and helium are the only resources we're currently consuming. The others are just being reversibly transformed.
Mining landfills is dangerous work, but it can be automated. Mining in general is dangerous work, but I suspect that landfills are even more unpredictable than rocks. Landfills contain smallpox virus, polychlorinated biphenyls, HIV-contaminated hypodermic syringes, and so on.
I think that current landfill mining is limited largely by aesthetic considerations (and the resulting laws), not pure self-interest.