There was no way to avoid it, since Nephis had to make space to allow other Supremes to establish their Domains. More importantly, it was almost a given that she would not be able to assist her subjects from the depths of the Nightmare — so, clinging to her throne served no purpose.
And lastly, there was the simple fact that no matter how their attempt to conquer the Fifth Nightmare ended, the Longing Domain would disappear anyway.
If Nephis perished in the Nightmare, there would be no more Changing Star to govern the sea of frail flames burning in the hearts of humanity. If she succeeded, though, she would not be a Sovereign anymore — she would be a Spirit. And Spirits did not govern Domains... not in the same way Supremes did, at least.
