I think the bitter sweet ending will be that Jon and Danny's baby takes the place of the baby he was denied, that baby then goes on to be the next Knight King.
It is said that Craster is the son of a ranger of the Night's Watch so maybe they need a son of a NW Ranger? Jon was a member of the Nights Watch etc.....
It would explain why all of a sudden the white walkers started forming to march south, basically following the baby they were denied.
Craster could have been fulfilling a purpose and a deal made to keep the KK at bay the first time, once Samuel broke that deal by taking the baby and no more babies were there to replace it then he was forced to follow to maintain his species survival?