Everything was answered perfectly. The "alt" time line was the afterlife.
Everything on the island was real, and everyone died in their separate ways.
"Some died before you, some died long after..." They were all reunited in the
afterlife at the end, because of the bonding and love they all shared with
one another. Jack died in the jungle with Vincent, while Kate, Claire, Sawyer
etc got off the island on the fixed Ajira flight and lived out their lives
until they too eventually died. Remember Kate saying to Jack "I've missed you
so much" ? She's presumably lived far longer than he has off the island, but
was finally joined together with him in the end because they loved each
other. Just like everyone else in the church scene in the end loved each
other. The Losties didn't have any purpose in life before they were brought
to the island. While they were there, they fell in love, sacrificed
themselves...their life had purpose.