Why was Amy Pond's son never brought back? He swore revenge on Dean, promising to kill him. It would have made for a powerful and unique storyline for the show. Jacob hunts Dean while managing to keep his vow of only killing him. Seeing that the murder of Amy Pond has come back to haunt Dean. The show really didn't give much to certain supernatural beings/characters. That beings that weren't human had a right to survive and thrive, and also that the hunters can be just as monstrous as those they hunt. Jacob could also put Sam in a personal/philosophical conundrum given how he was saved by Amy and he didn't even try to do something to at least honor her memory. Or Jacob could have knowledge of siblings who weren't human and were forced by the brothers or by one of them to kill one or the other despite their love. But the show is human centered and mostly spits on beings who aren't human, through inconsistency, ignorance, or something else. If they had done it, they probably would have messed it up and made Jacob another straw man situation. It's sad.