Every story ever made is a variation of some other story. The same goes for music, art, or any creative work. The details are what vary and make a piece of work "new." Nostalgia sells, but that's not all that is considered. I agree with what AngelCityOutlaw said. I mean, take Tangled, Tangled takes Rapunzel and makes it its own. Tangled is still very much Rapunzel, but it has a considerably different flavor, which prevents it from being "Another telling of Rapunzel" while still keeping the core themes and narrative intact. FF7's remake has a rich background to pull from. I'd say that there's plenty of room for a successful take on the original story, while being true to the themes of the first. The new FF7 will be different, but that doesn't mean it won't be good. Just different.
Whether or not it will sell well is something else entirely. I get the feeling that fans will buy it regardless of how it's received. The Hobbit trilogy sold well and those movies were major flops for hardcore fans. I went to see all 3 movies even though I had issues with them. If I had a PS4, I'd probably get FF7's remake, even if it was reviewed poorly because the idea of wandering around a setting as interesting as FF7's with modern graphics is really appealing.