Maybe one Seph (the one before going nuts) will go against Mother's will and protect the planet, because that's how the cycle can continue, and as long there is a planet there's a story, and Seph is part of that story.
Which is why Aerith says: "You're wrong... This isn't how it's supposed to be".
Of everything she knows, she expected your usual Sephiroth, but that one is different (because it's showing interest in protecting the planet) but she doesn't buy it.
As long there is a planet, there is a story that can be remade countless times with Seph and Cloud not ending.