You know, Serena, if you were right, you were right for the wrong reasons. Sixteen years, after all, is twice the length of the run of a regular hit show. She was the second longest serving junior ADA in the history of the series, having served three complete seasons and half of a fourth. Whatever your opinion of Serena Southerlyn and her outing, she is still a confidant, assertive character with a successful career her recent firing notwithstanding who has been watched by millions of Americans for four years. Serena played her sexuality close to the vest, but her apparent discomfort as a prosecutor in civil rights and more notably in cases regarding gay rights cases is very apparent in many episodes leading up to her final episode. You're a superb attorney; you ought to be involved in cases that feed your passion.
Legit, before I knew I was gay I had a picture of Mariska Hargitay taped up on my wall above my desk. Knows her way around a body, strategically buries the lede. The haircut, the lack of interest in Chris Noth, the time she got hit on by that female mechanic! Her departure was noteworthy due to a surprising  conversation between Southerlyn and Branch in the very last scene of the episode.
Edit Did You Know? I'll upvote if you can give an episode number or a youtube clip. NBC declined comment except to say: Casey is number one that is for sure. Haas directed the great "Angels and Insects," among others. There were very few details during her run on the show regarding Southerlyn's sexuality.