They probably could pull it off if that was the sticking point, they could reuse and tweak some FFXIV models if that was a problem.
Heck Yoshida, the chara designer for Bravely Default and Second is also known for working on FFXIV after all.
Strangely enough I think it's more that it really was the style they were comfortable with.
they had to go back to the drawing board from VII onward for FF (with FFVIII completely ditching any chibi models).
BD2 having realistic proportion would be a dramatic shift for the franchise and I don't think that's what they want for it more than anything.
With Trials of Mana for example, they have far more realistic proportions in 3D while the original spritework was barely a step above chibi FF games.
A shift that dramatic is probably closer to the Crystal chronicle games
that started like this
And had a big cinematic finish like this
Although Crystal Bearer really looks like the least cohesive game out of the franchise.
Is this a Secret of Evermore joke?