I don't think the dagger becomes powerless just because Rumple realizes he's the Guardian. I think he's going to face a similar situation to the one in 3x11, where the only way to defeat the villain is if he dies too. But he's still the DO, and he'd die as the DO, thus he would never be reunited with Belle (Or so he thinks). He'd still make that sacrifice simply because it's the right thing to do, and that
completely selfless act would break his curse, and that's when he realizes he was the Guardian all along. Or maybe Belle explains everything to him when they reunite, because of course she would know.
