I get what you are saying, it makes a lot of sense, but Jacob is immortal, right? He doesn't need a replacement, he just wants one so he can finally die. He created the premises of his own death by bringing people to the island. MIB apparently doesn't have the power to bring people to the island. So the island doesn't really need a protector. Sure, we can argue that people can stiil come to the island without Jacob's interferance.
But I'm wondering about one thing: If Jacob wants out, why did he not tell Richard "kill me, and then become my replacement; you do want to live forever, so that you won't go to hell, right?". Yeah, Richard is not a candidate, but couldn't Jacob make him one? Doesn't J make his own rules by now?