It's hard to say. Life is unpredictable. Personally, I believe that we will be the engineers of some event that knocks us down, and topples our modern day society and technology levels, crippling us as a species, leaving us largely powerless. And then, when we're at our weakest point, another event will take place, one that we won't be able to combat, and it'll knock us out for good. The two events that will most likely knock us down will either be the environment going out of control on such a scale that all we'll be able to do is simply flee or do our best to cope. And the second, could still potentially be nuclear fire. At both points, it will be a war of attrition that wittles us down to nothing, until the last man standing stands no more.