I bet Sony did force Raimi to do Venom,if we are getting Venom.
They probably thought,Ok this is the last one for Maguire,Dunst,Franco and possibly Raimi,so we'll end it BIG.
And they probably think that the only way to top Doc Ock,is to have Venom as the next villain,along with Sandman of course.
Sam probably agreed,as long as he got to write how it's done.Which he has.Sam and his brother Ivan have written the story for SM-3.