No doubt a FFVII remake would sell very well, but there's one issue: most people already have played FFVII and therefore know the story, which is the main thing about JRPGs. I don't think half of the people that played it will buy the game again to replay it with better graphics. I mean it's 80 hours or so.
That there's lot of hype for it on forums like this is understandable; it's one of the best games ever and the graphics are very meh nowadays. That doesn't mean that the wider audience is eagerly awaiting a remake.
Still it would definitely sell 5 million at least if on a platform with significant userbase.