To sum it up
-There is no way we will get so many SHs
-Yamato joining at the end would be pointless since her original dream of seeing the world would be offscreened
-If Oda wanted her to join then why would he change her reasoning at the very last second? There are only 2 explanations that I can come up with: The maximun cope where Oda wants to make film red canon, so he kept Yamato away for now and will bring her back very soon, or the realistic one, that she wont join period