Sorry, i read your explanation incorrectly. your definition of a destination race justifies what you said, however, I would define it completely different and say I don't agree.
The IM brand is a beast until itself and they have shown they can pretty much do whatever they want and the lemmings will keep showing up. Personally, i don't believe it is because people want to 'race' a great course, however, that is just my beliefs. Don't kid yourself though, there are MANY more reasons why IM races sell out other than it is well run event.