I think you're dramatically overestimating the likelihood that artifacts would survive in recognizable form for millions of years. I do think we can be reasona

I think you're dramatically overestimating the likelihood that artifacts would survive in recognizable form for millions of years. I do think we can be reasonably sure that there was no advanced technological society let's say 50,000 years ago. But 20M years ago is another story. 5M years might be another story. Whether you'd find fossils of an advanced technological species at a point in their history when they might have a population of billions seems incredibly contingent on unknowable things about their culture. But the question of whether you'd find fossils of them from their prehistoric period (in which their population would not necessarily be enormous), or of their evolutionary ancestors seems less contingent, and very plausibly you wouldn't.