Returning to a teenager for Haibara wouldn't do her any good. She's too scared to do it. She has more friends now in the DB than anywhere else- she deserves to win back her childhood. If fate is part of DC, then Shinichi shrunk to bring justice to the BO at the risk of losing his love life, and Ai won back her childhood at the cost of having to help Shinichi despite personal fears. But she looks so cute with Mitsuhiko, and Ayumi is definitely not good enough for him.