Everything changes. Right now it is dark and serious is all the rage. In time the pendulum will start heading in the other direction. Fads come and go and at some point audiences will want superheroes that are a little more lighthearted.
I'd say there is room for both. Look at how successful The Avengers
and The Dark Knight
were. Two very different movies, but both incredible successes.
I do agree that there will always be room for both. I wonder what tone Batman will take once it is rebooted? It is possible that the next director may steer away from the serious tone of the Nolan movies in an effort to make it different and to put his own creative touch on it. Or they may make it close in style to the Nolan movies because they were so successful. There are many options.