I actually like the nano-theory of Devil Fruits, but there is a huge problem with it - if Devil Fruits are caused by technology (which can be replicated), why is there only ever one of each fruit? Wouldn't it make sense if the government knows how to make them for them to replicate the really awesome fruits (such as Kuma's) and give them to all their top officers? What if all the Navy Captains and above had fruits as good as Kuma or even just as good as Kizaru? Pirates would be in deep shit.