I actually agree with you 100%. My theory, now, is that what happened on the Island was real. I think it is instead the "future" to be an after-life dimention where they gather before "moving ahead". The reason why I considered the possibility that they were all dead is simply that they could all have died with the plane crash itself. But, I repeatedly discarded the option because it seemed to me unfit or too naive a solution, in one way or another.

Anyway, originally, I thought that the "future" was a secondary line of time originated by the event of Oceanic flight not crashing. But, that can't be true, because characters' past seem to change as well: Sawyer as a detective was not his choice after landing; it had been made before. Same goes with many other characters (the young physisists - forgot his name - becomes a piano player, Juliet goes from accepting a job with Dharma Initiative to being a doctor in an hospital, and many others)

I have to say that I liked, overall.

But, I think I am still craving for a bit more explanation. Just a bit more. What really was the Dharma Initiative? Who was Withmore, and what did he know? Why should the island be protected? What kind of creature really is the black smoke, in some more details? I understand their choice not to explain, but I would have wanted to know a bit, even just a bit, more.

And - but this is my fault - I am still a bit confused about Desmond's role in all of this. I got distracted there.

In any case, I am overall quite happy with the ending. The more time passes by, the more I like it. It is deep and touching, in many ways.