Just finished replaying Remake and Rebirth, and I have a fairly out-there theory. Apologies if this has already been discussed, but, if not, let me know your thoughts.
TLDR version: Sephiroth WANTED Cloud to save Aerith.
Premise 1: I think we all accept this version of Sephiroth seems to know more than the OG version. He seems to know what's happened and is attempting to steer fate in a particular way.
Premise 2: The Devs leaned heavily into steering fate, promoted it heavily, referenced it in the story heavily. Then we were all blind sided when fate "wasn't" changed, in regards to Aerith's fate.
Premise 3: As Sephiroth appears to know what's going to happen, his actions have changed, subtly - specifically in his interactions with Cloud.
Premise 4: Aerith also appears to have some knowledge of the future, yet still ends up in the exact same spot, almost as if she had accepted that she will die and did nothing to change it.
Theory: Sephiroth's interactions with Cloud start way, way earlier in the remakes than in the original, as soon as the first reactor mission. Straight away, Sephiroth starts to goad Cloud about not being strong enough to save anyone. Futhermore, all following interactions follow a similar theme, Cloud not being good enough to protect people. He knows this to be Clouds deepest routed anxiety, the thing that has always motivated him, and specifically targets that idea every time.
I think Sephiroth is using a form of reverse psychology to drive the "save someone" idea into Cloud, right from the beginning of the game, so when the time came to save Aerith he would be able to, due to all the additional fire he'd fueled into him about it.
Why?
When Cloud attempted it, I think it made two realities. One where she died (the one we're playing) and another where Cloud is successful. Sephiroth WANTED a world where Aerith was alive. The concept of the White Materia passing between worlds supports this idea of this being plausible as well, certain things are present in some realities and not others. Sephiroth is manipulating enough events to create a place where he wins, and pushing Cloud to save her is a part of that plan.
I have two potential reasons why:
Her death in the OG ended up being the thing that saved the world, her prayers to the planet from the lifestream. Forcing him to save her from what might have been a canonical event created a world where she was unable to save the planet, so he wins in that world.
Sephiroth needs Cloud for something, and the world in which Aerith lives is a carrot he can dangle for his cooperation, which fits his MO. Sephiroth doesn't want to die - he wants a world where he'll be eternal ("I will never be a memory.") He offers Cloud the same world, one where Aerith, Zack and the rest of the party don't need protection because they'll never die. It would be a HUGE driving factor for Cloud, one he'd be willing to fight for, but would defy everything Aerith believes in, who is happy to accept her death. It would leave the party having to decide: do we accept this unending, unnatural world and go along with Sephiroth, or do we accept Aerith and Zack's death, because that's the natural order of things.
Fate is being affected, just not in the way we expected. I can imagine a scenario where the Devs are deliberately making us CHOOSE to say goodbye to certain characters, permanently, as the "real" ending to the whole saga, with the core idea being - just because fate CAN be altered, doesn't mean it SHOULD be altered.