I was thinking more in terms of what Eliezer wants to portray rather than our world. “Rationalists should win” is one of Eliezer’s maxims, and setting up someone as “the guy who didn’t know the power of rationality” to win most of the time would dilute that lesson.

Also, MoR!Dumbledore is simply not irrational enough. Little of his success can be attributed simply to luck, and while he is not rational on a level with Harry, it seems at least that quite a bit of his success can be attributed to making hard decisions instead of just following instinct. For example, regardless of what really happened with Draco’s mum, Dumbledore’s behavior on this subject is indicative of cold calculation rather than wishful thinking. The same can be said on the subject of his brother, especially since he was so conflicted about it. Note also how much of his arguing with Harry is more about morality rather than reasoning.