As most of you likely remember, Rachel Taylor was the female hacker from the first Transformers movie. Since her character is already established, that positions her as an ideal replacement for Megan Fox. If they try to shove in a new character then it's going to feel tacked on and stupid, but if they make Sam's new love interest someone who's already established then it feels more natural. I think the audience would be more willing to forgive it if Sam was suddenly with someone else if his new love interest was a pre-existing character. Picture this: Mikela can't handle all the robots and baysplosions in Sam's life, so she packs up and leaves poor Sam in a rut. Later, Sam runs into that gorgeous blond techie that he met shortly after he got Bumblebee. Now that Sam's a bit more grown up, Maggie now notices him in a way she didn't when they first met. Being a little older and more experienced than Sam, she helps him move on, and the two of them have a spark. Being a government technician who works with the Transformers, Maggie is already accustomed to a dangerous lifestyle like Sam's, and can cope with it much better than Mikela could. Personally, even though the Maggie character's role in the first film was fairly small, I found her to be much more interesting than Fox's Mikela character. She was smart, inquisitive, and proactive, whereas Mikela was just kind of there. Maggie was not afraid of getting into trouble to solve the mystery, whereas Mikela just sort of followed Sam around. If Sam ended up with Maggie instead of Mikela it would be an upgrade IMO.