[Chris_Jay]

>What structures today are likely to survive long term?

The Georgia Guidestones come to mind. I've been quite interested in the pre-ice age golden age theory lately but I think the part missing from your analysis is that we're farther ahead on a chemical/particle tech branch than on a social engineering tech branch. We've essentially scaled up competing warbands to a planetary scale - a class 1 civilization we are not, and seem more likely to develop one on Mars than Earth.

Building the Georgia Guidestones probably cost much less in human terms than a comparably complex henge, but we lack the mindset to collectively see the value in it. To make a gaming analogy of your point, I would say we're objectively ahead on one branch of the tech tree but we overlook simpler technologies on branches we haven't explored yet, to the point of not realizing other civilizations were farther ahead in some regards while being behind in others.