At least for the Aztecs, this is in fact the case. The Aztec kept subjugated peoples alive to husband them to supply human beings for their sacrifices. The Spa

At least for the Aztecs, this is in fact the case. The Aztec kept subjugated peoples alive to husband them to supply human beings for their sacrifices. The Spaniards allied themselves early with these would be victims, who therefore helped remove the numeric superiority the Aztecs would have otherwise enjoyed. These allied tribes (see the Tlaxcala) then became a privilege class among the natives. The guns, and the horses, obviously helped - but we're talking 4 orders of magnitude advantage if the Spaniards had not allied themselves with the locals. Even pandemics cannot erase that advantage (and they take time to spread)