There will be spoilers for the revamped Scholomance Mists of Pandaria heroic in this post.
Arthas Menethil is dead.
He's not just dead; he's really, most sincerely dead. He is no more. He has ceased to be. Bereft of life, yes, but as Sylvanas Windrunner saw during her own recent experience in Northrend, he does not rest in peace. The Lich King lives on in the form of Bolvar Fordragon, but Arthas? Arthas is dead. No king rules forever.
And yet Arthas' hand reached far across the world before he died. Even before he became a Lich King, Arthas forever warped the world of Azeroth. Even before his soul was blasted and rent asunder by Frostmourne and he became a servant of the then-Lich King, the former Ner'zhul, Arthas destroyed a city by his own hand and his own will. What Arthas did can never be truly undone. As much as they hate him, the Forsaken of Undercity owe their freedom from the Burning Legion to Arthas' move against the Dreadlords following the Battle for Mount Hyjal. As much as she hates him, Sylvanas owes not only her current existence but the val'kyr that have twice raised her from death to Arthas.
Unlike Tinfoil Hat KYLs, this particular post is speculation and rumination without a theory to support or prove. It's musing on the nature of undeath, the role of the Lich King, the discordant notes and unreliable narrators of this particular part of the story. It asks several questions and does not have any answers to them.
Does Bolvar Fordragon sit alone atop the Frozen Throne? Does Sylvanas Windrunner rule herself? Can we trust the val'kyr's word? And if Arthas Menethil still existed within the Lich King, holding the Scourge in check, then what did Tirion Fordring destroy in the Cathedral of Darkness? What of Matthias Lenher?
I have no answers but many questions.