> People get stuck on the idea that the only way to determine there was intelligent life earlier is to find a super complicated artifact which obviously can't survive

Why is it obvious that none of them can survive? Ordinary bones can become rocks (fossils) that are preserved. If mining of iron ore is visible, things would have been made out of iron -- wouldn't a large structure made out of iron leave some trace? Or even a small one? Why are there dinosaur fossils galore, but not so much as a pre-human fidget spinner to be found?