I don't think LF sits the Iron Throne in the show, but I think there's a really high chance that when it show wraps it's foreshadowed that he will. I think there's almost zero chance that he gets his comeuppance. Unless it's in a way where Sansa essentially becomes the new LF. Like her meeting with him was the first time she actually took a step towards being his peer. Her laying out that she thinks he's scum, but then letting him live makes her seem like a weak little girl, and her letter writing probably continues this role, but this could be a sick student-surpasses-the-master arc. Any other downfall of LF has to involve good triumphing over evil, and that'll be stupid lame.