It seemed pretty straightforward to me, but obviously it wasn't for everybody. Just to be clear - thy did NOT die in the Oceanic 815 crash. It's not even a debate, as the producers have made that clear on a number of occasions. As far back as Season 1 and as recently as a month ago in a podcast they confirmed that the Losties did not die in the 815 crash and that they were indeed alive on the island (until they did die).
But for those needing further evidence:
-Christian telling Jack that it was all real
-Christian telling Jack, and I quote: "the time you spent with these people was the most important time in your LIFE" - in your life, not in your afterlife
-If they had all died in the original plane crash, they would have been all strangers upon their deaths, and there would be no reason for them to connect as such in order to move on
-The off island events that predated the plane crash (like Richard visiting baby Locke in 1954), but not interacting with him, would have no place in the story
There is so much left open for debate and interpretation, and that's all fun, it's just a waste of time to debate something that is definitive, like this. It's like debating if Jack's name was really Jack. Pointless.