for context, the 10 million inhabitants number is in dispute. some estimates for N. America alone go up towards 20 million or even 50 million (including Mexico).

Part One: Numbers from Nowhere

Mann first treats New England in the 17th century. He disagrees with the popular idea that European technologies were superior to those of Native Americans, using guns as a specific example. The Native Americans considered them little more than "noisemakers", and concluded they were more difficult to aim than arrows. Prominent colonist John Smith of the southern Jamestown colony noted as an "awful truth" that a gun "could not shoot as far as an arrow could fly". Moccasins were more comfortable and sturdy than the boots Europeans wore, and were preferred by most during that era because their padding offered a more silent approach to warfare. The Indian canoes could be paddled faster and were more maneuverable than any small European boats.

The contrasting approaches of "High Counters" and "Low Counters" among historians are discussed. Among the former, anthropologist Henry F. Dobyns estimated the number of pre-Columbian Native Americans as close to 100 million, while critics of the High Counters include David Henige, who wrote Numbers from Nowhere (1998).

https://en.wikipedia.org/wiki/1491:_New_Revelations_of_the_Americas_Before_Columbus

The population debate has often had ideological underpinnings. Low estimates were sometimes reflective of European notions of cultural and racial superiority. Historian Francis Jennings argued, "Scholarly wisdom long held that Indians were so inferior in mind and works that they could not possibly have created or sustained large populations." In 1998, Africanist Historian David Henige said many population estimates are the result of arbitrary formulas applied from unreliable sources.

https://en.wikipedia.org/wiki/Population_history_of_the_Indigenous_peoples_of_the_Americas#Estimations

it's worth noting that Tenoctitlan at around a quarter of a million inhabitants was likely the 4th largest city on earth at the time, with a population larger than current day Paris or Istanbul and had 4x the population of London. the spanish conquistadors purposely fouled it's extremely innovative water and transportation system, making disease rampant.

The plague thing is obviously bullshit beyond the first few decades. Why were there still Indian Wars until the 1920s if they all died of disease?

which is ignoring pox blankets,

Also the literally several dozen wars of extermination committed by the US