When the Spanish arrived, most of these terraces were either in use or very recently abandoned, as the first wave of the plagues had swept into S. America only a year or two earlier.
I don’t see anything particularly mysterious about a population of tens of millions in not particularly hospitable terrain creating massive structures to help them feed an expanding population.
I’m not sure what your last paragraph means. S. America is loaded with unexcavated cities, with ceramics, etc. That few Americans are familiar with them doesn’t mean they don’t exist.
First, I doubt that when the Spanish first arrived, they decided to wander up more than 10,000 feet, maybe just to look at what their ships looked like from their?
I’ve never heard of any speculations that there were “tens of millions” of people. I absolutely agree with that, but do you have a reference?
I am saying this is pretty much beyond anything we see in Europe or the Middle East. Just by it’s very scale, we have to acknowledge some kine of technologies that no one else has yet.
Plagues that repeatedly killed millions, like in this article explains:
Historical Review: Megadrought And Megadeath In 16th Century Mexico (Hemorrhagic Fever)
"The epidemic of cocoliztli from 1545 to 1548 killed an estimated 5 million to 15 million people, or up to 80% of the native population of Mexico (Figure 1). In absolute and relative terms the 1545 epidemic was one of the worst demographic catastrophes in human history, approaching even the Black Death of bubonic plague, which killed approximately 25 million in western Europe from 1347 to 1351 or about 50% of the regional population."
The cocoliztli epidemic from 1576 to 1578 cocoliztli epidemic killed an additional 2 to 2.5 million people, or about 50% of the remaining native population."