I think candidates are not for replacement of Jacob, instead they are functioning like cork, as Jacob said, impeding MIB to spread off the island. It is also part of the tacit rule that MIB can not kill them himself, just like he can not kill Jacob
Refer to Jacob's other quote "Someone will replace me" is explicitly saying that candidates are not meant for what we thought before. If it did, then he would have said "one of the candidates would replace me"
What do you think?