A master rationalist can still be persuaded by a good arguer because most arguments aren't about facts. Once everyone agrees about facts, you can still argue about goals and policy - what people should do, what the law should make them do, how a sandwich ought to taste to be called a sandwich, what's a good looking dress to wear tonight.
If everyone agreed about facts and goals, there wouldn't be much of an argument left. Most human arguments have no objective right party because they disagree about goals, about what should be or what is right.
One obvious reply would be to hire rationalists only to adjudicate that which has been phrased as a question of simple fact.
To the extent that you do think that people who've learned to be good epistemic critics have an advantage in listening to values arguments as well, then go ahead and hire rationalists to adjudicate that as well. (Who does the hiring, though?) Is the idea that rationalists have an advantage here, enough that people would still hire them, but the advantage is much weaker and hence they can be swayed by highly paid arguers?
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