Considering the rendering of the Devastator character was so memory intensive that it actually destroyed computers at ILM, I'd imagine that it was necessary for it to be destroyed so easily, because a full-on fight scene wouldn't have been possible to render.

BTW, if 1986 Optimus were just as callous towards Megatron as 2009 Optimus was to Demolishor, he would have shot him as soon as he pointed his rifle at him, instead of covering him like police do of a surrendering offender... long enough in fact for Megatron to stall and grab a weapon. IMO