I seems pretty simple to me that we now know that Jughead is the failsafe device that Desmond used to stop the end of the world in 2007.
My theory is that in the next couple of Episodes Jack tries to get Daniels Jughead plan to go through, but nobody, not Dharma, the Others or Losties are willing to take the risk of detonation. Either Jack compromises and lets the failsafe be built or the Losties are rescued to their own time before his plan is in place, but Dharma and the Others work out a truce on the swan to build the Swan and the failsafe and to leave it alone. This is why they did not bother with it during the Purge. This explains why no one ever tried the failsafe key before because they did not know if it would work.
My theory is that in the next couple of Episodes Jack tries to get Daniels Jughead plan to go through, but nobody, not Dharma, the Others or Losties are willing to take the risk of detonation. Either Jack compromises and lets the failsafe be built or the Losties are rescued to their own time before his plan is in place, but Dharma and the Others work out a truce on the swan to build the Swan and the failsafe and to leave it alone. This is why they did not bother with it during the Purge. This explains why no one ever tried the failsafe key before because they did not know if it would work.
