Even with Lucas' anyone can learn the Force thing he still sticks to the idea that some people are inherently stronger in their connection to it than others:

The Skywalker line is very strong with the Force, so Ben says, “I think we should protect the kids, because they may be able to help us right the wrong that your husband has created in the universe.”

So while you could train others, they still wouldn't be a match for those that are naturally more attuned to it like the Emperor and Vader.