Mr Light wrote:

Lord knows if your last movie made a billion dollars, something needs to change 
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It's driven by the toys, most likely. The Dark of the Moon toy line flopped compared with the previous two films, to the point that the last several waves of figures were canned in the USA. And really it's no surprise – we had three movies of the same characters with basically the same designs that Hasbro has tried to sell to parents over and over. At some point, the parents tell their kids that they don't
need a twelfth stentorian robot that turns into the same flame-covered truck. A ground-up redesign avoids that issue.