Rapmastermind
Superstar
This move actually makes sense for all parties. Will is being cautious with the roles he's taking, especially after the critical drubbing he got for AFTER EARTH. As you saw he dropped out of that some-what NEWISH director's first big budget film. He's being careful with his brand and wants a hit,. Michael Bay, re-uniting with Will and Martin to a beloved franchise is as close to a guarantee the movie will be a hit as you will get. Makes sense for WILL SMITH. Plus Bay is fond of the franchise and loves working with Will and Martin.
Not only that but Bay is really tired of "Transformers", what I mean by that is since 2005 when they started pre-production, he's done mostly only Transformers. That's 9 years of Robots. Yes it's printing him money something ridiculous but the only way he would do "Transformers 4" is if they let him do "Pain and Gain". I think he would of done "Bad Boys III" during that gap but once again Martin and Will's plates were full. Will got some "Hitch"-like Romantic Com coming out called "Focus" but your right, he's being very cautious. I think Martin hasn't done any movies again in 3 years. I'm sure Bay already told them he's doing "Bad Boys III" before Transformers 5.