What do you think would have happened had Megatron been left on the battlefield? Knowing how the Autobots are I mean. And considering that Prime would still have passed while he was presumably in their custody. I can see Kup or maybe Springer walkin up to him and ending it somehow after Prime went. Unless they had an "Optimus wouldnt want it this way moment". Would Megatron then have laughed in their face with his little strength, while shackled up the way Shockwave had him in the Marvel comics?