I know you said you recognize that, but unless we can show that this line of reasoning is coherent (let alone leads to a relevant conclusion, let alone correct) then there's no reason to assume that Eliezer's point isn't trivial in the end.
If what you're saying is this, then we agree: EY doesn't here present an argument that UCMAs are likely to be false, but he does successfully argue that a certain class of generalizations over mind-space are likely to be false (such as generalizations about what minds will find persuasive) along with the assumption that a UCMA will fall into that class.
If that's the line, then I think the argument is sound so far as it goes. UCMA enthusiasts (I am not among them, but I know them well) will not accept the final assumption, but you may be right that the burden is on them to show that UCMA's (whatever 'compelling' is supposed to mean) does not fall into this class.
Alternatively, we could just posit that we're only arguing against those people who do accept the assumption, that is those people who do take 'compelling' in UCMA to mean something like 'immediately persuasive', but then we're probably tilting at windmills.
I suspect that our beliefs are close enough to each other at this point that any perceived differences are as likely to be due to minor linguistic quibbles as to actual disagreement. Which is to say, I wouldn't have phrased it like you did (had I said it with that phrasing I would disagree) but I think that our maps are closer than our wording would suggest.
If anyone who does think they have a coherent definition for UCMA that does not involve persuasiveness (subject to the above taboos) wants to chime in I'd love to hear it. Otherwise, I think the thread has reached its (happy) conclusion.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.