While you're correct in a simple short-term scenario, there is one very big hole in that argument: if the composition of the incoming trash changes over time. W

While you're correct in a simple short-term scenario, there is one very big hole in that argument: if the composition of the incoming trash changes over time. Which it does. Plenty of things were not recognized as valuable at the time they were thrown away. There's the classic example of low-background steel, but there are more arcane things as well: an old high school teacher of mine bragged that he salvaged a large amount of discarded old wood planks, worth as much as a nice new car! Even common items may help factor in to making recycling old dumps economical (until recent years, an enormous amount of consumer electronics, containing precious metals, were simply tossed in the trash.) One way I could easily see this becoming big fast is from some sort of law enforcing a heavy carbon tax or pollutant tax. Steel production is highly polluting and currently nobody pays the true price of things (including their carbon footprint.) If a government takes one of these steps - which I could totally see happening - then people will definitely consider mining landfills.