Uhura will leave when she realises that all her "new development" is as little more than a whiney love interest. She'll be replaced by Cadet/Lieutenant M'Ress.
Who, in turn will leave when she realizes she's treated as little more than a glorified switchboard operator and the subject of LOLcat
"Do I look like %^&* Garfield?!"
"Well, you have gained a few pounds lately..."