I was thinking about this a little further, and if they did it, I thought it'd be a good idea to have Optimus and the core characters around just for a prologue chapter, before they disappear. That way they can still include Optimus in the advertising. Makes it more marketable and recognisable. Then once that's out of the way, they could focus on Magnus's crew and some of the lesser known characters. Then near the end, ... they receive a mysterious transmission.