You stated :
"One thing in particular has always bothered me about Ancient Astronauts - how they managed to not leave a trace of the infrastructure needed to support and maintain their technologies."
Correct me if I am mistaken, but I thought that the initial question referred to Ancient Aliens, not Ancient Astronauts.
Why is it, that when the word "alien" is used, the discussion immediately involves spaceships?
Why is it, that when the term "UFO" is used, the discussion immediately involves aliens.
Why do aliens need to be astronauts?
Ancient Astronauts may have needed infrastructure, but Ancient Aliens would not have necessarily needed anything.