I don't really see the issue with this.
After all, action games for instance frequently have stories that are a fraction of that of most RPGs.
The reason they're still stand-alone games at relative short lengths though, is partly because games like Uncharted are much more technically demanding game-play-wise and that requires more development-time and resources.
Since this game is obviously going to be action based, and they're making things to scale (and Nomura babbling on about stressing the ability to interact with the environments) it's completely reasonable to expect even the rudimentary story of FFVII to stretch out into three volumes - because last time I checked, with voice-acting etc. that would still be longer than the story of all the Uncharted games put together.
Not saying they won't bloat the game etc. but from a development perspective this is pretty basic.
But the way I see it - if they're going to create the entire world, and fix its broken infrastructure (add roads, railroad tracks, expand cities/towns to actual resemble real cities/towns capable of sustaining an actual population) then that offers of lot of natural padding to the game.
If FFVX is anything to go by, we're looking at a game where it's going to take a lot of time to get from point A to B, and we're probably looking at the introduction of mini-quests like monster hunting and NPC fetch-quests to fill up that dead-space etc.
I'm not worried about stats etc though. Legend of Heroes have already proven without a shadow of a doubt (although even games as early as Suikoden 1 and 2 did this) that you can have game series with transferable stats.
As for release date -
Releasing the first game within 2017 should also be reasonable simple. The game has already been in development for over a year. By late 2017 we're closing in on 3 years.
To put this into perspective - FF8 was made in little over a year.
This game is being outsourced to more than 2 companies besides the work being done at SE, and it's obvious based on the game-play trailer that they are in part using resources from the FFVX development process (Clouds "free-running"/crawling through the ruins animation is obviously ported over from FFVX as it is identical to that of Noctis).