I suppose. I would recommend playing it before assuming too much about tone. I can't imagine how they could make the plot much more darker.

I just feel like people exagerrate about changes when what's happen with FF15 isn't anything new or out of the ordinary. Games change radically over development, even moreso when the creative cycle is strecthed over years.

Yeah, it is possible it could have scored higher if they delayed it to work on it more, but at this point, it doesn't really matter. No point in wondering what could have been.

Since it's just a thought...

One more year would have made it worse.

If that team can't come up with a decent way of presenting a story after three years(as you say), another year certainly wouldn't have helped.
This was a half-assed attempt at a masterpiece and they should leave it. Build a new team with fresh ideas and start working on the next iteration. And please for crying out loud, get that story telling in order...

If you play it steadily now the added content will be out before you finish. The stuff they are adding is near the end of the game.