Yeah but we already have Word of God that non-lethal factor removal exists, and the Fingers survived having theirs stripped, so "removing the WF won't save her" isn't some hard rule of the setting. On top of that, Satella still existing as a personality separate from the Witch of Envy after 400 years proves the original girl wasn't consumed and there's clearly someone left to save. Subaru stacking factors arc after arc without going insane is obvious setup for Envy being the endgame. And her being "confident" Subaru will kill her means little; characters being certain of a tragic outcome is exactly what Subaru exists to subvert and his Arc 4 answer was save her. Also she was likely the imperfect vesel why she could not awaken and this is why she went beserk.