Just don't think that adding an extra boss-fight against sahalanthropus at the end would make much difference.
It really won't. People are acting like there is so much cut content, but so far this is the only thing that we definitively know was cut. The chapter 3 peace could have just been a rearrangement of title names. And really, what would a 3rd fight with ST-84 have brought to the game? Not much really. Sure it would have been a new area to explore, but it would have just been Africa-esque, and we don't know how big the level would have been.
I'm not sure the game was rushed either. I think it was just badly paced and poorly written, and no amount of extra content is going to make up for an otherwise luke-warm story.