Why does everyone assume the "they're dead" explanation?

[woogie]

I have not been on the forums in a while and I was bored here at work so I thought I would check out the old foums. After browsing the threads hear I can see that a lot of people are saying the flashsideways is not an alt timeline and that everyone was just dead... am I missing something? Was there some extr info released that I did not see?? I saw the finale and the ending just went white. As far as Im concerned they stayed in the FS, in an alternate world. They just all had knowledge of their alternate lives. They were as much dead as they were all alive. The FS no matter how you interpret it was an alternate story for the characters that happened parallel to the original story. So am i missing something or are people just discussing their own interpretations because as it stands my own interpretation is still valid.