When he was first offered it, he said he wasn't interested because he didn't like how Native Americans were treated in Westerns.

Although that ending did bother me. I was hoping that they too would let Hardy go, their violence over since they recovered their lost daughter and, in the end, the "success" of the adventure was helping bring the father and daughter together, which he could never do with his son.
As it stands, it does say that, while his focus on vengeance kept him alive, it was his saving the girl that ultimately saves his own life, but that moment (and much of the final act) felt like a standard movie cliche.

Rather than a thinkpiece, though, I hope someone would just try to talk to Inarritu and Leo about it because I'm sure they could give a thoughtful answer on it. The portrayal of Native Americans seemed very important to both of them at the Q&A I saw.