I've always assumed we'll end up re-processing landfills when it hits an economic tipping point: better tech emerges, raw sources are exhausted, etc.
There's a story in Junkyard Planet of this exact thing making someone wealthy when a product that was treated as waste became valuable to the steel industry, and they knew where to find it in the dump.
I wonder this too, eventually robots will pick apart the landfills once there is technology to deal with it. Tons of metal goes into landfills that could go to scrap yards, etc Altho when they cap off landfills they become a ski resort or a subvision.
There's a story in Junkyard Planet of this exact thing making someone wealthy when a product that was treated as waste became valuable to the steel industry, and they knew where to find it in the dump.