If humans were sent back to the Stone Age, rebuilding modern technology wouldn't take millennia—it would be impossible. The accessible resources needed to start over are already gone.
When the first Industrial Revolution began in the 18th century, it was initially fueled by water power and wood before transitioning to relatively accessible underground coal pits.
These early resources provided the energetic stepping stones required to build basic steam engines—which were first used to pump water out of those very mines—allowing miners to dig deeper and build more complex machines.Over the last two centuries, industrial societies have strip-mined those accessible layers. Much of the easy coal is gone. The shallow iron deposits are largely exhausted. To reach essential metals today, mining companies must construct mile-wide, terraced craters and process vast amounts of rock to extract trace amounts of copper. Extracting modern oil requires drilling thousands of feet beneath the ocean floor or fracturing deep shale deposits.
A reset society would face an impossible paradox: extracting deep-earth resources requires advanced steel machinery and diesel fuel, but building that machinery requires those exact resources. Wood and charcoal can forge basic iron tools, but they cannot sustain an industrial revolution or reach the temperatures required for mass steel production without rapidly deforesting the planet. A second civilization would find itself permanently locked out of the industrial age because the ladder of easy resources has already been kicked away.
Without fossil fuels to power the Haber-Bosch process—which produces the synthetic fertilizer that feeds billions of people—a post-reset society would also be severely population-constrained. Stripped of the energetic density of coal and oil, and unable to build the infrastructure required for solar or nuclear power, a second iteration of human civilization would likely plateau indefinitely at an agrarian, medieval level of technology.