Eloise knew that things could be changed. This is why she prevented Desmond from buying the ring. She knew that he had to turn the failsafe key, become immune to electromagnetism, unplug the cork of the island which makes MIB mortal.
If he hadn't done that, they'd all die.
oh, and Widmore also knew all about Desmond, which is why he did his best to keep him and Penny apart. When Desmond goes into his office to ask for her hand in marriage, he immediately tells him he's unworthy.