The Inca Civil war devastated the empire , followed by the advent of disease and then by the destructive Spanish conquest . If you take Pizarro out of the equation, I'm not entirely sure that the Inca's could have simply picked up the pieces. There were other conquistadors, after all and the other European powers wouldn't have stood by and ignored such an opportunity. Perhaps the Inca's would have lasted another century, but no longer, I'm afraid
You've got the order wrong. The Inca Civil War happened because of disease, which killed both the Sapa Inca and the heir apparent. If you have Pizarro out of the equation, especially during his first expedition, then disease spread can happen very differently, and there's no guarantee that either the Emperor or the heir die to it. Pizarro showing up was also a hammer(a lucky one at that) smashing a glass house when it was at its most fragile, so yes, other conquistadors would have had far less luck on average. Then the magical cooperative single-goal-minded Europeans are going to inevitably take turns at conquering the Inca? Come on now. So your determinism, while predictable, is horribly off-base and reveals(like far too many posts in this thread) that everyone's got a hot take on the inevitable fate of the Inca but are straight up peddling misinformation. To quote markus meecham, put down the Jared Diamond already
Are you implying that they would not give it another shot? Some basic math. If it took Pizarro and 167 men to conquer the Inca, than 20,000/168 says that the Spanish could launch nearly 120 expeditions! Of course, it would be more realistically be a dozen expeditions. I admit that. Are you implying Pizarro was a "Great Man" and not someone who wanted glory? Than he can get in line because there were plenty of people like that back then.
One can discuss the plausibility of a PoD or the ramifications of it. I have not doubted that this scenario is plausible. This whole time I have been discussing the ramifications. And I say that it is out of character for the Spanish not to give it another shot(s). The Inca may last decades but they can't keep the Spanish out. They will be coming in as soldiers, missionaries, mercenaries, and "advisors" very soon. They will have an enormous influence on the local government. And having a large influence gives you the chance to take the reigns yourself in the power vacuum if the government falls.
And frankly the only way to survive smallpox during that time period was to be exposed to it. It is better for humanity if all the natives get exposed in one go and are given the chance to develop an immunity to further outbreaks than leaving pockets of non-exposed populations to die later or catch it and spread it back to the rest of the population.
Ignoring the obvious(that a large number of those men if not the vast majority are needed to hold down Spain's existing empire, function as settlers, traders, administrators, have already settled down or conquered their share, etc.), you're pulling the number of expeditions out of a hat. As for Pizarro, have you
read what he did? The amount of luck he had was ridiculous. A worse guide through the Andes could have easily led his entire expedition to death, a few days' delay could have seen Atahualpa himself unable or unwilling to attend to him personally leaving his massive gamble in taking him hostage, a more cautious Atahualpa would have seen the encounter at Cajamarca not result in an unarmed army showing up as a show of force, etc. So yes, he is a 'Great Man' in the sense that no one else other than maybe his more malicious and incompetent brothers had the cajones to do what he did.
As for being out of character, read up on how the Spanish launched expeditions. It wasn't simply a matter of convincing a few dozen men hanging around Havana to follow you. You had to authorize it with the local governor, and with the king of Spain, and chances are the governor was actively trying to snub you of glory and wealth. The Spanish were already overextended, and had a standing order to limit their expeditions. And if Pizarro fails, then the Aztec conquest becomes a once in a lifetime situation at best, nothing more. And if you get lone Spanish bands(that as a result of being unsanctioned and borderline treason are far smaller and less organized), chances are, the Inca CAN effectively repel them. The Inca can absolutely keep the Spanish out or engage with them on their terms.