they were all dead the whole time

Lost was a victim of the "mystery box" style of writing, in which the writers when conceptualizing a long-term mystery only considered it's introduction and left the resolution for later. This lead to some mysteries never gaining a resolution, some gaining a completely asinine resolution, and even some that previously were given a satisfactory resolution being retconned for an asinine resolution (ex. the whispers were at first the others being weird, later retconned into being the dead people in purgatory on the island). Add to that, the characters became unrecognizable and stupid in the final season. All that made for an incredible downfall for a once awesome show.