Vishi is in a coma, Nia is near death, and Filly is severely injured.
Among these three, Vishi is the one who should be abandoned the most. Her death wouldn't make Ash shed a single tear—if he did, he'd surely scold himself for being an idiot—but just like why Ash wouldn't kill her, her death would mean she would be reborn from the soul cage, return to reality, and might even break free from the ruby chain.
She's like the clichéd villain in a fantasy novel that can only be sealed away. Her death would only sow great peril for the future. Binding her to Ash is equivalent to sacrificing him to benefit the whole world.
Moreover, even Ash has to admit that Vishi is a complete treasure. Her knowledge, her Magic Realm, and even her very existence hold immense value. If Ash wants to go further down the path of a mage, there's no better guide than Vishi.
