perspective: Well, I’m not I see how you can disagree with me, seeing how vague my conclusions on the Americas were :D.
Seriously, you’re certainly correct the damage would have been less. I’m just not sure it is quantifiable just how much less it would have been.
Take Mesoamerica for example - Let’s assume just for the sake of argument a pre-contact population of 25 million around 1520, an estimate I consider a reasonable compromise figure. By around the the turn of the 17th century ( I don’t have the exact census date, I think it was in the 1590’s ), that population had dropped to less than a million. Now, the clear consensus from historical records was that disease, principally smallpox, but also measles and cholera, accounted for the great majority of that mortality. Brutal forced labor conditions also exacted a toll, however. The question is how much of a toll it added.
We also know that in contrast the Andean plateaux, starting from what was almost certainly a significantly smaller base population ( say 7-11 million ) pre-contact, had probably a slightly larger population around 1600 than Mesoamerica. Why? Well, you could argue that labor exploitation was less severe because the Andes were a less readily exploitable region ( mines aside ), due to local geography. This is true. However what is also true is that Andean population were discontinuous in distribution compared to Mesoamerica, divided by ridgelines into separate communes, with little non-State controlled traffic between them ( whereas independant traders appeared to have been common in Mesoamerica ). Insomuch as smallpox, measles, and cholera all depend on human vectors, this discontinuity almost surely slowed the spread of disease, especially as the gaps between population centers would have actually increased as communities succumbed to disease, slowing the spread even more.
Also although the first epidemic was the most severe, new epidemics periodically burst forth to decimate the native populations again and again. Were these the result of continual reintroductions by new waves of European conquistadores? Or once introduced, did the diseases incubate in native immune “Typhoid Mary’s”, to explode outward from within?
See, not knowing the answers to the above, I find it hard to estimate if the population of Mesoamerica in 1600 if the Spanish hadn’t set down roots. It would have almost certainly been more than >one million. But would it have been 15 million? 10 million? 2 million? I dunno. How much would it have taken to knock out city-centered polities like that of the Aztecs? Not much, I’m guessing. Their’s in particular was in essence a tribute-based City-State, dependent on their formidable military machine to generate the outside tribute necessary to feed Tenochitlan’s ( and associated near-satellites like Texcoco and Tlacopan ) population. Given cities were likely going to be hardest hit due to population density issues, the Aztecs would have surely folded. The Inca state might have fractured into pieces as it tried to contain epidemic centers. etc. The ensuing political chaos would have likely caused even more loss of life.
Not saying they wouldn’t have still been better off. They probably would have been. I’m just not sure how much better off they’d be, is all :).