Well, I think someone can certainly go wrong, but there's a remote possibility that an MI movie could work. You've got the piratey, voodoo-filled Caribbean world, and a good writer could create a passable story with Guybrush and company on an original adventure. As you pointed out, MI probably has the largest fan base of all the LucasArts adventures (though I'd say Sam & Max is the most marketable, I'd prefer any possible movie for those characters to be animated, and under the control of Purcell). I'm not sure that fan base is substantial enough to make the movie worth funding, but MI is something that could appeal to a mainstream audience if handled correctly, so I don't think the idea, at least theoretically, is all that prepostrous (though I still don't think in a million years it will happen).
And of course along with the negative side of the POTC situation which I've aleady discussed there's a positive aspect as well. After all, POTC was a huge success, and the numbers could mean that audiences are all for comedic pirate adventures that prominently feature the undead! I agree with you that people would be sold on MI once they got into it, but I'd be concerned about people being cynical about a possible movie simply by observing the marketing. It may just seems too similar for some people and could drive folks away. But yeah, the minute people read into the situation it wouldn't be a problem. The thing is not everyone would read into it.