From what I saw on Sat. I beg to disagree - all the parks were slammed and attendance was up at all four parks. We could not get away from the crowds. Cannabalize - no. Add more space for more people - yes. Think of what happens when you add on to your house - you get more stuff. More room = more people. A 5th and possibly 6th park would only add to the space that they can fit people in.
As far as your observations on people staying longer, I think that it can go both ways. The average trip to WDW seems to be about 3-5 days for people across country and a weekend for people who can drive to WDW in one day. Personally, I go every weekend due to my pass and stay about once a month. And I can tell you that most visitors that I talk to go to all four parks and one of the competition while they are there. It is easier for them to float around and see everything than to plan one day, one park.
And don't get me started on the cruises.
Sorry for a scorching response, as I have not read the last 100 posts, but this statement needed clarification from someone on the frontlines.
-J