ok the scenerio of the autobots interferring with Earth matters due to the decepticons poses interesting thoughts on our own society, our society could possibly already be manipulated by alien races - anything is possible these days. We are a collectively insane species - why ? - the world is simply not at peace and we will never be at peace till we raise our level of concious awareness so your stuck in a circle of unconcious stupidity - optimus could point this out - some people even believe achieving world peace is impossible - but if you look closely at that statement you will see they are living a life that is self fulfilling their own belief system. I think even though this movie is mostly aimed at people who wont think much it would be a good message to send out from the autobots.
As for story line goes, they could continue this on a deeper level , that the decepticons have already been manipulating the secret government society sector 7 who knew about the decepticons coming back.