I think they did that to set up the possibility of saving Aerith. Which I think is a good idea. They made her such a great and loveable character in the remake , her death is going to be absolutely devastating, far worse than in the original.
They also did it so they can include Zack, who obviously wasn’t on the original. People need to realize that a lot of FF7 material has been released since 1997, they just aren’t remaking the first game, they want to include the whole FF7 compilation.
A third thing is that they needed a real final boss for this installment. “Saving” Sephiroth for the next or even the third game years down the line wasn’t an option.
Well said, if they can find a way to save Aerith I am all for it, just as long as it’s done right.