I used to believe this logic too but seeing the crowd pics makes me reconsider. It's not too difficult to believe those crowds are from people who knew Mansion would be open. Who are those people? Avid Disney fans, Passholders, and CM's. They are probably NOT your average once-in-a-lifetime family visitor. I mean, I'm sure they are components, but they are not the core of that group.
Hence, that would imply a Local/Passholder/CM force that Disney underestimates at WDW. We here at DL are always told that Passholders are too large a component at DL but that Passholders are significantly diluted at WDW. Maybe that's still true but, as I say, those pics imply that there IS a significant 'locals' market that is being overlooked.
That would furthy imply that overlays could indeed be successful, I would think especially at a Park like Magic Kingdom. And I tend to doubt that a family visiting for their 1st time (or once in a long while) is going to cancel their vacation plans b/c HM is down for a month. At worst, maybe they'd reschedule if a) they don't like HMH, or b) they really want Mansion to be open.
And, incidentally, our Holiday overlays have become so popular that some guests schedule their visits to make sure they see them. Holiday crowds for these attractions are consistently huge during the overlay periods. SWH on average has longer wait times than at any other time of the year. I think it's partly the Holiday and just the novelty of something being 'special' during a particular time of year - a new experience.