If the recent years are to be of any indication, this trend will only get more and more pronounced until 2019 when the film is supposed to hit the theatres. With that in mind, it’s possible that Brad Bird and the rest of the team are simply looking into the future, and planning accordingly.
The director does have a lot of experience in the industry, and he should know best how to approach the development of a project that spans over such a long period. It’s important to approach the production from the point of view of someone who will watch it several years later when it’s released, and not now.
Brad Bird does seem a bit secretive about the whole thing in general, and the director has shared relatively little about the progress of the film so far. That could also indicate that he’s planning to take some risks and doesn’t want to spoil the surprise for his fans.
Or it could simply mean that the film is still in a volatile state and not much has been set in stone. Considering that it’s reportedly already in the animation stage though, that would be a bit strange. But Disney are the kind of studio that can afford to make serious changes in the middle of a long-term project if they determine it vital to its success, so only time will tell what’s truly going on behind the doors of the studio.