This kind of calculation is useless, since there were so many resets (and wars and floods and fires and catastrophes, etc.) in history where the world started again with an extremely reduced population.

I agree, it is not a great argument. You would have to calculate all of the deaths due to wars, pestilence, famines, etc. The Black Plague alone was devastating to Europe's population. "The Black Death, a devastating pandemic of bubonic plague in the mid-14th century, killed an estimated 30% to 60% (often cited as around 75 to 200 million people) of Europe's population, wiping out huge portions of Eurasia and North Africa and profoundly altering society."