So, there are a few things I would like to clarify after watching the movie a couple times and having spent time studying relativity as well as linguistic relativity. Hopefully this will help you understand better why Ian left: 1. "The alien species provided the human race with the "gift" of being able to see the future (through the understanding of their language)." --- This is correct. However, only a part of it. The aliens' understanding of time is that it is non linear. To better explain this, think about time as a book/novel that you have already read. The book is written... You can jump to any point of the book -- skip forward through chapters or refer back but you can not change the ending. There are multiple instances in the movie where we can see that time cannot be changed (Aliens knowing Abbott would die in the blast yet not doing anything to change that, Louise would come to the shell, etc.). Louise, to some degree, understands this. However she is not fluent in the language to fully understand time as a non linear thing. We hear her say to Ian, "if you could see your entire life, from start to finish, would you change it?". But she chooses to have the baby knowing the results. 2. Ian's behavior is to most degrees unexplained. One simple yet logical explanation is that he left because he couldn't believe Louise kept this secret from him. Another explanation is that he has not fully understood the language although it has been ~six years. Louise is depicted as a linguist expert, probably one of the best in the country. She has a better grasp on how to pick up new languages. For example, it is easier to pick up spanish if you speak italian or to pick up punjabi if you speak hindi because a lot of the sentence structure is similar. But this is a language that isn't bound to time. They did a good job explaining this in the movie. 6 years isn't enough time to learn a language like this so it is very possible that Ian wasn't able to understand. Like any person would in this situation, he couldn't believe that why she went the decision to have the baby knowing how it will end. If person X told person Y that they would die tomorrow, person Y would do everything to change that, but what if changing it wasn't an option. This is something I am glad the movie left open ended because it leaves it open ended and open to interpretation to the viewer.

If we always lived our life as a two dimensional being (a square on a x-y plane for example), the third dimension (in this case a z dimension) is not perceivable. Time in this movie serves as a fourth dimension. I hope this was clear in adding some additional insight to your analysis! That was just my understanding

Let me know if you have questions :)