Eliezer, I think your argument is flat-out invalid.

Here is the form of your argument: "You prefer X. This does not strike people as foolish. But if you always prefer X, it would be foolish. Therefore your preference really is foolish."

That conclusion does not follow without the premise "You always prefer X if you ever prefer X."

More plainly, you are supposing that there is some long run over which you could "pump money" from someone who expressed such-and-such a preference. BUT my preference over infinitely many repeated trials is not the same as my preference over one trial. AND You cannot demonstrate that that is absurd.