I believe that if the Man in Black is successful in his plan, then "reality" for everyone we know will become the alternate world we are seeing in these flash-sideways. No one will fulfill their purpose or destiny. Jacob has never touched these people so they never did what they were supposed to do. Their lives and relationships are not what they should be. When Juliet said "it worked" upon her death, I believe she is now "existing" in this alternate reality, and that if the Man in Black is successful, everyone else will die and also begin living their alternate realities. Thoughts?