Population bottleneck. Technology requires certain population levels to maintain. If the human race were choked down to say, half a billion, I bet we'd lose the ability to sustain modern semiconductor manufacturing. 50 million and kiss any new aluminum goodbye, less than a million, and blacksmiths start having trouble teaching mining and smelting iron ore from generation to generation. Less than a few thousand? the finer points of starting fires get lost. A few less and keeping a fire banked gets a little iffy...
Very interesting indeed! It’s true that when the population decreases, so does the infrastructure, and I imagine if the population got really reduced, even very basic parts of the infrastructure could disappear. That certainly explains things like the disappearance of the knowledge of, say, plumbing and central heating and water, which the Romans had, but which vanished with the demise of Rome.
There was a British author named Bryher who wrote historical books for kids in the 1940s or so, and one book was set in Roman Britain. The first sign of the collapse of the Empire was that they stopped repairing the roads, and then finally nobody knew how to put together the stones to get that type of pavement.