It's nice to think that the movie will have good legs, but an opening weekend is something that is impossible to get back.
For example, the average NFL Sunday afternoon broadcast draws less than 5 million viewers. The NFL's weekly showcase, Sunday nights on NBC, draws 20 to 30 million. The Super Bowl gets over one hundred million viewers. Years ago I read an article in the New York Times that stated that something like half of the Super Bowl audience had not, in fact, watched any other football that year.
Studios try to create an event with an opening weekend, something like the Super Bowl- an unofficial national holiday. It becomes the thing to do- and particularly the thing to do for an audience of millions who have no particular interest in Batman (or Spider-Man, or The Avengers, or Harry Potter or Pirates, etc.).
Once this tragedy happened, and also the Warner marketing machine went dark, that audience was gone forever.