I have heard it said, though, that if you love someone you let them go. And I think that even though Jack kept prodding Ennis to live that "sweet life" with him, he ultimately understood that this was the way Ennis always would be- running away from him and coming back. Jack lets him go, Ennis always returns. Ennis even tried to come back that November.
IMO, if Jack had lived that would have been decided in November. Jack would never have
wanted to let Ennis go and would not have if Ennis could have brought himself to agree to even a compromise. But if not, he was weighing alternatives for what he wanted to do with the rest of his life. In the movie, his relationship with Randall is an open space to be filled in by the viewer: would Randall have divorced his wife to be with Jack? In the one scene where he appears, he doesn't appear to be too happy with her and her seemingly bubble-headed chatter is often rather disrespectful of him, especially since she's talking with two people they've just met.
Not that Randall could ever have "replaced" Jack; as if any person can ever been replaced. But I've heard people say so many times, after a divorce or some other event where they've put someone out of their life: "I love this person so much and always will but I just couldn't cope with them anymore." Or had run out of energy, or hope, or just time in the sense of getting older. Jack was close to 40, an age where he would have started realizing that this romance in a wilderness ghetto wasn't something he could build the rest of his life on.