This answer mentions that David Weber had planned for Honor Harrington to perish in the Battle of Manticore, a claim which is also made on Wikipedia with a little more detail:
Weber also feels that in the later books in the series, she has more in common with Admiral Horatio Nelson than Hornblower. Weber revealed in the foreword to Storm from the Shadows that it had originally been his intent to kill her off in the Battle of Manticore, thus further echoing Nelson's death (in his greatest battle, Trafalgar) and have the emerging Mesa problem be dealt with by the next generation, specifically her children. When writing At All Costs, he decided instead to keep her alive, and move the Mesan-related events up, to be her problem.
From this it seems clear enough why he'd originally intended for her to die, as a parallel to Nelson's death while winning the battle of Trafalgar. But why did he change his mind and let her survive the Battle of Manticore? Was it due to feared backlash from fans, his own fondness for the character, practical narrative imperative, or what?