As I said before, look to the history of Portuguese Africa to know what will happen in America. They can conquer many strips of land, even in Mexico but probably will not enter deep into the new continent, their better chances are in Argentina, the Islands and in North America. If the butterflies dont mess everything and the european powers still develop to be the great powers they were in the 19th century America will fall to Europe at the same time as Africa and Asia.
Argentina? Increasing the amount of indigenous people who survive isn't gonna help much when most of them will end up being Mapuche or allies. Same goes with North America, where Europeans might find it difficult to get a foothold if a lot more East Coast natives are surviving.
To be fair, there's also the Mississippians to consider. Less losses suffered might really, really help their case, albeit at the same time their centralised nature makes them more vulnerable to Spanish conquistador efforts than other groups. If Spain still takes Mexico, then I can't see the groups north of the Valley of Mexico and perhaps the Puebloans too not falling to Spain...and at that point you're gonna introduce the horse onto the Great Plains and now you'll have thousands of more Plains Indians who aren't dying of smallpox surviving, adopting horses, and ready to raid you. But to be fair, without smallpox, the settled American Indian groups will be stronger too.
Really, from what I get, Indian diplomacy and native alliances are going to be HUGELY more important TTL, since EVERYWHERE the numbers of any potential enemy will be far higher, much more higher than your local colonists. And Indian diplomacy was often expensive too--it might be even more expensive this TTL with larger populations. For Spain, if the Pueblo Revolt still happens, they might just not even bother coming back to New Mexico. England will like
wise have issues colonising the East Coast. If King Philip's War was by all means the most threatening Indian War in US history, something like it is gonna be far, far worse TTL, simply because of the numbers involved.
Conversely, Indian slavery will be far more viable since there will be much larger numbers of potential slaves, although because of the inherent issues of Indian slavery, Africans will still be more important for slavery. The main destination for Indian slaves (perhaps many from Mississippian civilisations) will be the Caribbean as OTL.
So Montezuma was a 'glorious leader of Best Aztec' type? I didn't know that.
According to native chroniclers writing a generation or two after the fact, he was insanely incompetent when dealing with the Spaniards. To his credit, I've seen it suggested that Montezuma was not actually as bad a ruler as those native sources state (these arguments tend to rely on the natives placing all the blame for being conquered on him), but the sheer ridiculousness of a few hundred guys landing on the coast with such a riduculously beligerent mission (plunder wealth and convert people to our faith in the name of our king, who by the way we're effectively rebelling against through disobeying his appointed authorities), then finding massive amounts of allies to help them destroy what by all means is the most powerful state in the region makes me strongly suspect that something was desperately wrong with the Aztec Triple Alliance and Montezuma did not help it one bit.
Peru is more difficult, but still has issues with regular civil wars among different Inca claimants (even if Huayna Capac isn't killed by smallpox, he's not immortal; he has to die sometime), which opens up vulnerabilities, and the Spanish still have a strong technological edge (which was probably more important in Peru than in Mexico, as the climate is more conducive to wearing metal armor). Besides, after the wealth of Mexico, it's going to be really easy to keep recruiting would-be conquistadors for future expeditions, so they can always keep trying.
The Inca conquest is almost borderline ASB, and after two or three times, which by all means might as well fail, people will realise its a death trap. Spain's best idea (for any other European state wanting to conquer the place) is to find a way to keep causing civil wars amongst the Inca, but how many were there aside from the famous Atahualpa-Huascar struggle? Finding ways to provoke rebellions (Ecuador's a good spot for this at first at least) is good too. Problem is, this implies a peaceful co-existence as well as regular trade to create the links to make this intrigue to begin with which is also a great way to allow the Inca state to modernise and thus make conquistador expeditions always, always fail.
It wasn't just smallpox, influenza and Measles were major players as well. Measles has a awe-inspiring R-naught of 12-18. One person will infect 12-18 people. Once it is an unprotected population there is no stopping it and it has a remarkable fatality rate.
It certainly is, and it's a shame that smallpox tends to dominate these types of discussions. Measles alone will kill HUGE amounts of people, and that's not including any other disease that gets introduced. I'm pretty sure that after smallpox, it was the number two killer out of introduced diseases in places like the Americas, the Pacific, etc. There's also the effects of measles in pregnant women to consider, which has long term effects, along with the long-term debilitating effects of measles in survivors. Rubella is even worse in terms of birth defects if it occurs in pregnant woman, leading to a long-term societal cost. I'm not certain to what degree congenital rubella syndrome crippled populations in places like the New World during epidemics--probably just added to the misery, really and helped make the other effects associated with epidemics even more destructive.