Pick any metric. Human rights? We got rid of slavery. Technology? we mechanized farming, first with steam engines and then with the first practical internal combustion engines, we discovered how to fly. Science? Maxwell wrote his equations, Einstein explained the photoelectric effect, proved atoms existed, and overthrew Newtonian physics, Max plank made physics quantum...

....by what metric wasn't it some of the most amazing years in human history?

This was all done by a population of ~1 billion people. Who actually were way less productive that people are today. If the human population dropped much below 1 billion, I might be worried. But IMHO ~1 billion seems to be a sweet spot.