The argument that I've heard goes something like this:
If when I die I merely cease to exist, and the world as we know it is ultimately doomed. That is, at some point the galaxy we live in will either collape or come apart, then how does my life have any meaning? What would it matter if I died today or 30 years from now? At some point everything I have ever done will be destroyed, so what is the point?
I remember reading a bit about Carl Sagan, he stated his belief that he would cease to exist at death, yet he said his greatest sadness about dying was that he wouldn't get to continue his research. He was asked why his research mattered if humanity was ultimately doomed and he really couldn't provide a good answer.
I'm kind of curious. In your scenerio, earth is doomed also, and some on this thread say the end is imminant. So what is the point? According to your point of view, God knows our each and every ultimate end, so here we are struggling against the inevitable like fish in a barrel, fearful of a fate that is preordained. So what is the point?
I am playing Devil's advocate here, but my point is that your question cuts both ways. And you falsely assume that people run their lives on the basis of imaginary answers to juvenile questions.
So what? I don't see why that should cause anyone to dispair of living his life. The fact that it's brief makes it all the more valuable.