The devs have talked about how they want the remake to more fully represent the Compilation of Final Fantasy VII, those being the spin-offs like Advent Children, Crisis Core, Dirge of Cerberus, and the short stories. Aerith's fate is rather set-in-stone in the sequels, and necessary within the narrative. They're adding stuff, but only for padding and to create more ties to the compilation, which didn't exist when the original game was out.
A similar case would be when the Hobbit films added Legolas. He's from Murkwood, as stated in the Lord of the Rings trilogy, but because the Hobbit was written first, he didn't appear in it since he didn't exist yet. But the creators of the Hobbit films had no such excuse, and had to add Legolas by the necessity of continuity. If Aerith's fate is changed, it will throw the entire continuity out of whack, the exact opposite of what the remake is trying to do.