Lost Finale: What I took from it all.
They died. They died. They died.
They died back on September 22nd, 2004 when the plane 'crashed on the island'. The show was not about the island or all the mystery, it was about the lives led by the characters who were on the plane. The island acted as a platform to give these people a chance to redeem themselves, forgive themselves and others, etc. I think it was simply a figurative course-correcter so that these characters could decide on where they belongs, Hades, hell, nothingness or heaven, nirvana, valhalla, etc.
The only way I'm clear of this, is because a. we end right where we started, and we're told out right by the ghost of Christian Shephard that they are in fact dead and b. at the end credits they pan over the untouched wreckage and beach of the Oceanic 815 flight. No camps, no foot prints, no life. It was exactly what we expected of a plane crash on a remote island - no one survives. Somehow, Lindelof, Abrams, Cuse and the like were able to get us to hope for the unbelievable and built a wonderful world around those hopes and the lives/deaths of what would probably be fairly uninteresting average folk whom may have died in a plane crash.
All the supporting characters who weren't on the plane were illusions created from our main characters memories of life off the island - and they were also created by the island as part of this guidance/correction system it tried to use to get these people 'off the island'. I think Jacob and the Man in Black are the most representational of this as they were obvious metaphors for good/evil faith/nihilism and darkness.