I hope youre aware that none of this is a particularly satisfying response to, well, any of the issues, at least in my eyes. If you cant even imagine what the next stage could potentially look like, if not with metals, why are you so certain that there even could be a next stage? Civilization is not the norm, if you removed humanity from Earth during the early stone age, theres no guarantee at all that we would have a similarly advanced civilization of, say, gorillas ruling the world by now. Yes, it may be a "quirk", but its also immensely complicated, requiring the culmination of a huge number of largely unrelated factors coming together to create the ideal environment for society to happen.