The discourse and instruction is over, and Mormon returns to the more mundane information of when he might write again. Mormon has been speaking of repentance, and that triggers his concerns for the lack of a repentant attitude among the Nephites.
Mormon sees their end. No doubt this is not the first time he has made such a statement to Moroni. When Mormon writes this, he writes it as a fairly common understanding, not as new information. It certainly does not get the space that the baptism of little children receives, so it is simply a confirmation of something that he as discussed with Moroni previously.