Let's revise that sentiment: most of the things Nomura is famous for (which are: story, character design, battle design) would already have been more or less complete at this stage in the game's development.
I think that a lot of people look at a long development period and assume that it is reasonable to say that. But that's not always true, especially if a large and ambitious project is developed over a long period of time with the idea and even core concepts evolving over time.
Let's consider these three particular points - story, character design, battle design.
The story in a RPG also dictates the overall scope and scale of the game. Is it likely that a story outline was completed many years ago, and a screenplay for most if not all of the cutscenes finished? Sure. But isn't it equally likely that if the scope of the game is redefined at various stages of development, that the story outline and even entire scripted scenes would have to change? In a stable and well managed production, there would be a content freeze once the team has determined exactly what they wanted the game to be after pre-production. Clearly this hasn't happened here since every 2 years or so we hear Nomura talking about huge changes, and eventually the game even changed platforms and title. So I wouldn't be so sure that the story is completely done either.
An example I will put forth is FFXII. Matsuno stepped down as director and left early in 2005. The game was in development for about four years at that point. Matsuno also well known as a director who focuses on both story and gameplay. Yet based on all the interviews after the game was released, it is very clear that they added writing staff to the game after he left and had to restructure some stuff and write new scenes. Why? Wouldn't the entire story be complete at that point? Shouldn't Matsuno have completed the story? Did they not like some of what he wrote? Was it simply incompatible because they had reduced the scope of the story and needed to make things more cohesive? We might never know the truth of the matter, but I'm sure a lot of these questions apply to FFXV as well.
Next we turn to the character design. Yes, this is usually one of the first things that should be completed in terms of pre-production for the visuals. But again, this production hasn't really been "normal" so to speak. We've seen character designs even for the main characters change a little here and there over the course of development. What's even more interesting to me is that usually Nomura shares singular credit for character design on games he works extensively on, but when the game was reannounced as FFXV last year, the design credits attached to the copyrights of the game are different. They now read "CHARACTER DESIGN: TETSUYA NOMURA & Roberto Ferrari". So there has clearly been a substantial change which the company felt deserved an amendment to design credit. Again, this is evidence that even now there could be ongoing design changes for one reason or another, so I wouldn't say that it is certain that whatever we see really represents some sort of absolute perfect vision Nomura had for the character designs from the start.
Then we get to battle design, which is actually the trickiest part of all. For the games Nomura directs, he usually has significant input in the gameplay design, yes. For many of the Kingdom Hearts games he comes up with the concept for a lot of the battle gimmicks and sub-systems. But he also leaves it to the actual battle directors and planners to flesh out his ideas into actual playable systems. In the case of XV here, we've seen signs that they've been experimenting a lot with the battle interface and mechanics over the years. For an action RPG, the most important thing for the gameplay is that it has to be fun. Sometimes you can have a good idea of what you want the game to "feel" like, but it can be very challenging to actually implement that into something that plays well. If they've been having trouble with that during the development of the game, then I don't think it's certain that Nomura's ideas for the battle design has been fully implemented either. Tabata is also a very gameplay-oriented director, so with him taking over completely, depending on how much he has to work with, his ideas could well be replacing Nomura's, especially if that's what is needed to get the game done.