Well, yeah, but in the original trilogy the Force was mystical. The fact that only some people can use it doesn't mean it's something physical, it can be in the soul or whatever you want to call it. It's just like in other fantasy stories only some people are born to be wizards and you don't need a rational explanation of that. How lame would a similar concept be in say Lord of the Rings or Harry Potter? "You see, Harry," Dumbledore said, "some people are muggles because they don't have enough midichlorians in their bloodstream...". The midichlorian explanation dumbs the whole concept down and totally destroys its mystical nature.