They sort of have all the assets to save time as well, I mean the story is done (unless it's changed up, maybe minor translation correction here and there).
I could see a 2017/2018 happening to be honest, not like they have do redo everything from scratch (development wise, they have a lot they can use from the get go).
No.
People say, "Well they have the game design, the story, the music, the character designs, that's already most of the assets, it shouldn't take too long!" Those people don't know how little of the game production the above is. That stuff can be put down on paper and be a bible for the rest of the development team and time, but actual time, resources and effort are put into the creation of visual assets (character models, enemy models, environments, objects, etc.), the animation of said visual assets, the development of the game engine, the implementation of assets into the game engine, the implementation of the game design into the game engine, other implementations (minigames, cutscenes, etc.), and a hefty amount of time is spent just fixing technical bugs, much less say things like gameplay and balance playtesting.
Even if they are doing "static backgrounds" style of the original (or like Pillars of Eternity), they don't have the original assets for that anymore. So those have to be created anew.
Having the original design and story means very little, like maybe one month shaved off pre-production if they're rushing it. Square can definitely exert a massive amount of effort and resources to get a game done in two years, but it will come at a high cost not only financially but also from the other projects in development in their Japan studios. That's the story of FF14: ARR, and ARR development started with much of its HD assets available. It sucked up most of the resources at SE Japan for two years and probably got a bunch of other projects cancelled. And then there's the question of whether or not they can do it for a freshly built single-player title, which by example FF15 is actually demonstrating that they can't do it in a timely fashion.
On the other hand it might be possible for them to share object/environment assets between FF15 and FF7R, so there's that for saving time and resources.
That's why it's optimistic to believe in 2017 release. Ultimately whether or not they can hit that year is dependent on the kind of game they're going to remake it into. If, for instance, they do FF13-corridors for FF7R I would say SE Japan has the ability to do it. Ha.