And disease immunity is built up over time. The Incas could lose a half or even three quarters of their population, but if the Spanish are held up enough they'll build up immunities and rebuild their population and empire.
I seriously doubt that disease immunity can be built up easily. Continental USA was having virgin-population epidemics among some American natives all the way into the early 1900s i.e. right up to vaccination era.
In fact, the less ongoing Spanish presence in Peru, the less resistance the Quechua will have. If contact is infrequent, it's far more likely that every time there IS contact, it's followed by an epidemic.
Seriously, I think people are really overestimating the effect of a few years here and there and really underestimating systemic differences between the populations on the two shores of the Atlantic. The Inca civil war was triggered by uncertainty in succession following the Inca's sudden death (possibly from the very same smallpox).
The Aztecs were putting a lot of pressure and isolating the few remaining powers in central Mexico, without having the manpower to finish them off forever. I bet one of two would gladly jump at the chance to ally with just about anyone else to overthrow the Aztec dominion.
And there could easily be other Malinches, there's always people who fall outside their society's protection and must make their own way to survive and prosper.
While none of these things necessarily WILL happen, any and all of them CAN happen, regardless whether the first contact will be in 1492, 1500, 1521 or even 1600 on the dot. I'd even say I'd expect the writer who decided to save Mesoamerican civilisations using Columbus' non-arrival as a POD to put a fair bit of explanation as to why it made such huge difference.
And I'd almost expect the reasons to have to do more with Africa, Asia, and Indian Ocean trade than America itself. If Spain misses out, the Portuguese discover the place first, and find nothing much more fancy than brazilwood, Spain might decide to focus on Algeria instead. Portugal by itself is too small to pull off as much settlement as Spain did, so that gives you time to change things. You'd also have to keep England out, Portugal alone might not be able to...things like that, basically.