The candidate isn’t there to replace Jacob.
This is just a quick thought of mine. I don’t necessarily believe what im writing because i am of the opionion that everyone in Lost is in some form or another lying so i don’t really know what’s going on anymore.
ANYWAY, according to Jacob, the candidates are there to replace him and therefore to keep MIB trapped on the Island. i don’t think that this is true for a few reasons. firstly, who was protecting the Island when Jacob was off it seeing Hurley, Jack, Kate etc? Also, i think Jacob would have got someone in place well before MIB indirectly killed him, if keeping MIB trapped was his main priority.
I don’t think its about keeping MIB trapped, i think it’s about proving him wrong (as Jacob stated in the S5 finale). This is what i believe the candidate’s purposes is; to show that even against all the odds, man can remain uncorruptable. Unlike how Claire and Sayid have been tricked by false promises, the Candidate will be the one who will be offered everything but refuses it.
For example, Kate might die (Jack’s main love in Lost) and MIB may offer him things like being reunited with her and a second chance with his father for which Jack will turn down this offer.
IDK, this is just a wild theory on my part written quickly just off the top of my head.