I don't think they all died at the nuke. I think the nuke merely created the flash-sideways universe, which was out of time, and which served as a gathering place for them all as they died. Jack's dad said they all died at different times but that there was "no now here (purgatory/flash-sideways)".
You're right. I forgot the rest of Christian's explanation.
what's your take man i'm just trying to see different point of views not necessarily anything else
Mainly, what people have said here; they're all dead at the end.
As cookie quoted, Christian said: "There is no 'now' here." They've gone beyond the reaches of time (which is interesting, when you think about it, considering how important time was throughout the series). Essentially, at different points throughout the course of their respective stories, each character has passed on (some characters died at points that we, as viewers, didn't witness); Jack died where we saw him die; Kate and Sawyer escaped the island (presumably) and died at some later point; Hurley and Ben remained on the island as its "protectors" after Jack died (which is why Hurley told Ben he was a "great number 2"); and so on and so forth.
Before they met each other, they were all lacking something; they were all "lost" (apologies for the tacky pun). It was only after they discovered their relationships and love for one another through the island that they felt whole; so, in the end, they created some place where they could all be together again. It's a sappy ending, but it really serves the emotional purpose of seeing them all reunited. Personally, it's what I wanted.
The character who has intrigued me the most throughout the sixth season is Desmond. He was "enlightened" initially by Charlie, who convinced him that there was some perfect form of love calling out to him. Then, after Desmond was convinced, it was revealed that Charlie actually had to be convinced all over again. I'm still trying to understand why exactly Desmond was the enlightenment figure (other than the obvious philosophical influence on his character); but then again, that's part of the fun of the show.