I suggest reading the work of Jared Diamond especially his other books besides, "Guns, Germs and Steel".
When sea level rose at the end of the Pleistocene as the massive, mile thick glaciers in the northern hemisphere melted it isolated the hunter gather groups on the Chatham Islands, Because these were groups of only a dozen or two individuals, the pressure to forage for food made it impossible to feed someone whose function was technological. So they lost knowledge of sewing, fish hooks and other very basic technology..
Ore tenors are very low today, but I doubt the assertion that a landfill is higher grade. But doubtless there will be those who prospect for a buried washing machine with the same fervor that others have sought gold.
Basic technology developed in Mesopotamia because there were readily exploitable deposits of copper, tin and iron. And there was an agrarian society which was capable of producing more food per agricultural worker than was needed to sustain them.
Those of us who understand technology and can repair things will be very valuable. But our numbers will decrease because there will be few who have the free time to learn the skills.
I spent several months reading all the books on the history of technology in my library. My initial intent was to write an essay in response to the "Doomsday Prepper" crowd who are hopelessly naive. After several months of study, I abandoned the project. There is no solution. The folly of human society over the last 100 years or so has cast the fate of the species in stone. At some point in time, for reasons no one can predict, over 90% of the species will die. It is an unavoidable consequence of globalization. The most likely cause will be a disease such as influenza disrupting the delivery of food. But any number of geological events such as a caldera collapse event at Yellowstone, an asteroid impact or a basalt flood like the Deccan Traps would also serve.
One of the great ironies is that those who are at the bottom of human society barely subsisting, will be the majority of the survivors.
The exhaustion of readily accessible resources will then lead to a slow decline into neolithic levels over a period of several hundred years. Most of the scrap will have been reduced to discolored spots on the ground after 200-300 years.
In the end, the human race will join the uncountable millions of other extinct species who have populated the earth. The conceit that humans are special is born of hubris and will be our undoing. We are, without question, the most advanced and powerful animals that have ever existed on the earth. But the unavoidable consequence of being born is death for an individual and extinction for a species.
That which created us and all those who went before us will also destroy us and replace us with something else. This has demonstrably been the way of things for several billion years.
It makes me a little sad, but it also makes me savor the magic we have created over the last 400 years. It took over 200 years for the calculus to become useful in engineering. Even as late as 1900, calculus had very little to contribute to the construction of a locomotive at the Baldwin Works. But gradually, gentlemen amusing themselves at university produced a sufficient body of science and mathematics, that engineers could use it in design instead of relying on trial and error.
Once that happened, engineering took off like a rocket. Commercial demands created by engineers led to more science and mathematics and we are now able to solve mathematical problems I was taught could not be solved when I was in school.