The buy rate for Wrestlemania 19 would seem to dispute that theory. Yes, there will be a set number of people who will buy Wrestlemania no matter what, but you'll get a larger number of buys by putting on matches that a greater number of people will pay to see.
Wrestlemania 19 only got a low buyrate because it was in Seattle. And because of the War in Iraq. And because of the popularity of Spongebob. And people didn't buy Booker T as a credible challenger. And Fabolous got arrested and couldn't do his freestyle battle with Cena. And Angle was a bad heel. And Lesnar wasn't ready. And they didn't promote Hulk/Vince enough. What are you HTQ, a mark?