While it's fun to speculate what if's, we have to remember, it's hard to say for certain what would become of mankind in the event of such a thing. Yes, I like to think we could rebound from something like that, but I am not sure we could. Would humanity devolve into a Mad Max kind of anarchy? If so, then we probably aren't going to rebound anytime soon. Yes, electronic devices can be remade, but what if there is no power grid? Seems our first priority might not be getting our cell phones working, but more along the lines of basic survival. Whatever calamity wiped out 2/3rds of humanity, is probably still having some impact on the planet, I would guess. It might take years to see any return of normalcy in regard to climates, tides, etc., depending on the event itself. Massive and multiple volcanic eruptions could block out the sun and cause temperatures to plummet, and all the green stuff to stop growing! What then? How do the remaining people survive a frozen and barren planet for several hundred years, before things return to normal? Could we survive that?
What fascinates me is the stuff we are digging up now, which blows our minds in terms of what we understood of ancient ancestors. They've found where they made things like batteries! Who really knows that our current civilization is the first to reach the intelligence/tech levels it has? The earth is 4+ billion years old, and has been destroyed numerous times by catastrophic events. How do we know for certain that we are the first true human civilization, or that some sentient beings didn't roam the planet ten's of millions of years ago? Of course, we haven't found evidence of this, but what if that's just because we haven't looked in the right place yet? What if all the evidence was destroyed by some cataclysm so great that nothing survived? Science actually keeps these questions open, and we can continue to learn new things about our past. It doesn't 'conclude' that one specific line of events transpired, and here we are!