The Man in Black has changed his plan and now intends to destroy the island using Desmond’s unique gift. Perhaps he succeeds and the result is the parallel timeline where the Island was on the ocean floor seemingly sunken.
The proverb “Be careful what you wish for, it might just come true” occurs to me and the Man in Black causes his own destruction when he destroys the Island because their natures are intertwined. He is part of the Island.
Everyone seems better off in the parallel universe. They might not be perfectly happy in their lives, but they are on the whole happier and not suffering as they have in the primary universe. The only exception I can think of is Bernard and Rose, perhaps Desmond and Penny too, but there seems to be some hope for happiness for them in the parallel universe.