What if whoever has the dagger (and therefore controls Rumple) commands Rumple not to be the Dark One?
Nice idea, but I hardly doubt that the power would just go PUFF. One of the fundamental laws in Physics is that energy isn’t lost, just transformed and this is actually one thing sci-fi and fantasy writers/etc tend to obey. The power had to go somewhere! Either the person commanding or some sort of box or whatever. Plus, I don’t think it could be that easy. It would be funnily anti-climatic, though.