Perhaps its not such a strange thing to say if you don't think Mr. Potter knows about the prophecy, and are trying to correct his insubordination. In the following chapters, Dumbledore doesn't act as though he has decided Harry is unsuitable as a hero. Rather than trying to replace him, Dumbledore begins to confide in him.

Does Nornagest's explanation of Dumbledore's relationship with story-book reasoning affect your previous analysis? If you agree that Dumbledore feigns a story-book persona, rather than taking story-book logic seriously, then doesn't it seem strange that he would hatch such a plot? Note that his manipulation of the last battle in December is consistent with having realistic view of the world. Yes, Dumbledore did manage to acheive a "story-book outcome," but he clearly didn't expect this to happen---he had a contingency plan.