I read an essay about that last parting scene. One, it was the first time that Jack really opened up about his feelings for Ennis and vice verse. These two spent 20 years not really owning up to what they were doing with each other. They pretended, for each other anyway, that they were straight and somehow liked being together while not being "queer". But Jack's revelations about Mexico, about wanting a life with Ennis over anything and anyone else, wishing he knew how to quit Jack were in a sense new ground for them...new admissions, guilt, shame, etc. And what happens? Ennis crumples into a heap like the end of the first summer when Jack did leave him. He also gets enraged that Jack would be with other men. Women he could deal with b/c (a) it was "normal" and (b) it was meaningless to their affair. But Ennis' reaction shows that he really has deep feelings and who knows how he has shown it over the 20 years.
Two, Jack finally learns that Ennis essentially trapped himself into a life where he could not provide for his family. He took dead end jobs, instead of say the one at the power company, so he could quit on a moment's notice to be with Jack. He suffered in the eyes of his family and community by not being a provider...all for Jack.
Three, he's imprisoned by his sense of responsibility. When he got a divorce, it wasn't the freedom he thought it would be given that he now had child support. So the job became more important than before. He couldn't take dead-end jobs for Jack anymore.
Which leads me to four, the "what if" question (thanks for bearing with me): Jack dies just as Ennis is totally freed from all responsibility. Junior is 19 and has been child-support-less for a year. Jenny is at most a year away from falling off Ennis' financial responsibility. So after all the declarations on that last day, after knowing that a year from then, Ennis didn't "need" to keep that job anymore, and knowing that Junior, who he was closest to would be leaving, what if Jack didn't die and they got together in November as planned? What would happen if there was no reason for Ennis to stay in Riverton or work that job?
I think that Ennis would have perhaps gotten together with Jack more often but not moved up to Lighting Flat. His willingness to quit the job for Junior's wedding only showed one thing: Ennis learned from his mistake.