She told Desmond he doesn't buy the ring. He breaks it off, goes to the island, pushes the button etc...She said if he doesn't do those things, GOD HELP US ALL.
The same button he one day fails to push, crashes 815, passengers take over, decide to stop pushing that button. Desmond then turns the failsafe key.
Desmond was brought there so he could turn that key and become invicible to electromagnetism.
Is it safe to assume that Jacob visited Eloise off island as he did Widmore? Perhaps time travelling Jacob?