Who's giving up all of their power for one shot? There's no evidence that's what Gon is doing, apart from the opinion of his enemy who knows far less about nen than Gon. Moreover, from a purely practical point of view, how exciting will the rest of the story be if Gon loses his nen powers here? As far as I know, this isn't the end of HxH. Gon hasn't even met his old man yet, despite the fact he stupidly, or gonnishly, thinks Kite being dead is the end of the world.