If this was the case what you'd have is a once sith lord being artificially kept alive via cybernetic enhancements. Hmmm... where have I seen that before?
The fact that Grievous was half humanoid half robot shows us Vader was nothing new nothing special which is lame as hell. I don't dislike Grievous' name or his moustache twirling villainy I just dislike him being. It's kind of like wait we've run out of ideas let's make him like the villain everyone is watching these movies for.
Same with Dooku. A once respected and powerful Jedi becomes disenchanted and goes to the dark side. Please. But that's a whole other topic for discussion.