Show of opinions, then? I think our overpopulation will choke us; it wouldn't make us extinct, but I can imagine some great famine/disease/war/disaster befalling most populations and our numbers falling. IMO we should already be trying to colonise Mars or something, but most persons are too unconcerned with anything beyond the scope of their own lives — humanity's success, and indeed mere survival on a universal scale. Of course if we don't try to leave Earth, Sol and ultimately the Milky Way, we won't last forever. I don't care what will kill us off entirely, rather I find the notion that we should die at all a concern that needs addressing.