Observations[]
Looking back at the original Dharma station/button, there are allot of similarities to the current events. In both cases people are trying to stop whatever is in that cave from escaping the island. Smokey and the electromagnetic energy are both manifestations of some part of that source. If either one of them were to escape the confines of the island the entire world would be at risk. IIRC Smokey used Ecko's dead brother to attempt to convince not to hit the button. We now know why he was trying to release that energy.
The Dharma initiative created the first situation, so they assumed the original role of guardian. They created the Swan and kept watch over it until as long as they were able. Eventually the responsibilities were passed along to Desmond who accepted the role on blind faith. This role was eventually passed on to Locke when Jack didn’t have the faith to accept it.
The smoke monster was created/released when Jacob threw his brother into the cave. He assumed the guardian role, ensuring that Smokey could not leave. His original task, passed down by his mother, was to protect the light from people, he assumed both roles on blind faith as well.
Conclusions and Theories[]
Jacob knows that he will eventually need to pass along both responsibilities, but needs to find someone who has a good nature similar to how he was when he was younger. That's why he brings people to the island, and that's what the candidates are all about. He believes that people are good natured because he has no choice.
IMO Jack has already inherited Jacobs role. In the button days he was lacked the faith, but clearly this has changed. The turning point was when he jumped off Sawyer’s boat. Desmond’s remaining role on the island will have something to do with his ability to withstand the energy from the light/source. He will most likely have to enter the cave at some point to help out. Possibly to permanently close seal off the light similar to what he did at the Swan.