Because they're the same person? Gollie.
The Phantom really doesn't diminish Sheik's returning chances. While they may have slipped a bit, there is no reason to write Sheik off yet. We've seen a grand total of one distinct attack, maybe two or three in vague mid-animation screens. We simply have not enough evidence to argue between the Phantom and Sheik. They can co-exist in the same moveset. That said, Sheik's chances have slipped very slightly—not enough to be worried, but slightly. If Zelda's moveset remains identical to Brawl with the Phantom addition, it could become her new down special.
Impa's chances only rise when Sheik's fall. I mean, Sakurai could include both if he wanted, but that seems asinine. As an Impa supported, I'm excited about her slight boost in potential with this Phantom reveal (which I also wanted included as part of Zelda's moveset), so as far as my personal interests go, it's a win-win.
Just remember, Sheik and The Phantom are important aspects of Zelda's legacy. As such, they can both be included in her moveset.
ALSO! Considering the discrepancy between her "grab animation" and the summoning of the Phantom, might it be likely the Phantom is incorporated into her throws? In Spirit Tracks you controlled the Phantom remotely from Link. The Phantoms also appear and vanish in a cloud of smoke, so summoning them from the ether would fit into quick sequences. Considering the distance between Zelda and the Phantom in the one screenshot, if this is a throw, wherein the Phantom slashes the opponent, then the remote control could be a reference to the distance between Link and Phantom Zelda in ST.