“You can’t go wielding supreme executive power just ‘cause some watery tart threw a sword at you!”
He’s right. More right than anyone’s probably ever considered. And the reason you can’t go wielding supreme etc. is because who, in their right mind, would accept that as a source of authority?
If I came round to your house and said the Dude in the Fireplace handed me a magic butterknife signifying that I’m your new Emperor, you’d call the cops and they’d likely shoot me as a dangerous insane criminal armed with eight swords, an AK-47, clearly high on PCP, affiliated with Al Qaeda and probably communists, and with two kilos of cocaine that somehow magically fit in my back pocket.
If the sword was in the stone, and everyone was essentially in agreement that the person to draw it would be king, at least the sword therefore then imparts some sort of authority. Which is why it’d be worth your time to steal it, even if you’re carrying it around with stone stuck on the end like some kind of medieval rebar club.