I think the novelty might be wearing off. Im sure there'll be a crowd of people who'll see the 3D version exclusively but that won't represent the whole of the market. Its just a natural progression really.
For instance:
X number of people watch movies in black and white.
X - Y number of people care that the movie is in colour.
X - Y - Z number of people care that the movie is in 3D.
Its always going to be diminishing returns once you move beyond the basic experience. A lot of people buy games consoles even though they don't really care for the graphics improvements in order to play current/new titles. Just as a lot of people buy Blu Ray because its the latest standard, not because they really care for it or saw much need to upgrade.