Hello Glynn. You make me feel nostalgic here, as I had touched on this topic in my very first post in this forum back in early January. The thread discussed Ennis's very last words ('I swear') and I elaborated on this, with a reference to the screenplay directive for the final shot.
You will find quite a lot of other posts that touches on this. In a discusssion group I was fortunate enough to lead in early June, this subject was also touched.
I quote here the final section of my first post. You will find the last paragraph relevant to this discussion.
***********
My interpretation is this: it is Ennis' own affirmation of love -- and regret. Ennis has finally understood a deeper facet of love. This understanding shines thru when he surprisingly accepts his daughter's wedding invitation (he has at least come to a realization of life's true priorities). It shines thru when he sees all too clearly his daughter's blossoming love for her husband to be.
And it is this realization of love that both raises Ennis up and crushes him.
It is because with the realization of his own Love, Ennis also realizes that it is a bit too late for him and Jack. And the final shot outside the window in a way affirms all of this, for the last words of the screenplay calls to our attention a deep sense of BEAUTY and a profound LONELINESS (the 'vast bleakness of the northern plains') that comes with having loved and having affirmed that love all too late.
