I think that after having 5 actors in 8 films portraying Bruce Wayne/Batman going through many physical and emotional struggles, audiences want to see him have a happy ending. Nolan's trilogy alone showed Bruce Wayne's journey from beginning to end. I felt that Nolan could have made 2 great movies between the Dark Knight and the Dark Knight Rises about Bruce Wayne's career as a fully formed Batman instead of skipping ahead 8 years to a Bruce Wayne past his physical prime, but there was already 7 Batman films before the Dark Knight Rises and Nolan probably felt that the character's journey needed to end before the sequels became more of the same.
There was a Batman film in the 60's, one in the 80's, three in the 90's, two in the 2000's, and now one in the 2010's. If Bruce Wayne co-stars in Justice League movies and stars in Batman movies it'll be too much.