everyone did not die in the original plane crash
That is your opinion.
The way I see it, there are 2 ways to look at it.
1. The plane crashed, they all died. The events on the island was a test. Eventually everyone on the island 'dies.' They move on and meet up at the church and go into the light.
OR
2. The plane crashed, they survived. The island events happened in real time, and everyone eventually dies. They meet up at the church and go into the light.