It's not only improbable, but impossible for them to have been dead all along. Matt, you can tell anyone who says that to you that I said they're out to lunch:

1. Julliette wasn't on the plane.
2. Ben wasn't on the plane.
3. Penny wasn't on the plane.
4. Desmond wasn't on the plane.
5. Why would characters who were on the plane, die AGAIN? (Boone, Shanon, Jin, Sun, etc...)

And I'm just including people who re-gained their previous consciousness and were at the church.

I believe they gave a number of obvious nods in the final episode alone that confirmed that what had happened on the island was real. Including near the end where Ben and Hugo have the short exchange about being #1 and #2.

Did you guys notice that while the church looked like a typical Christian church (inside and outside), the room Jack entered into where he talked to his father (who coincidentally is also named Christian ) was full of decorations/artifacts from a number of different world religions.

While I also would have loved to have a few more elements revealed, I think it would have honestly taken twice as many episodes, if not three times to unravel them cleanly, so it didn't seem like they were just spoon-feeding you a cheat sheet. I already had the feeling a number of times this past season that they were often times rolling through a check-list exposing things to the audience. In the end I think I agree with the producers that this was, all along, mostly a character-driven story. I cared far more about the characters in the end than I did about the details. So much in fact that I didn't even think for a second "hey, what about xxxx" when it ended.

Maybe we'll see a story some day set in the Lost universe that will reveal everything else. Maybe a book.