I think Jack would have always mourned Ennis, but he would have made a new life without him, still being sociable, joining up with someone else, but the drinking would have become more pronounced and Jack might have become less productive, with no one to encourage him to be the warm loving person that he is. Like starboardlignt said, Jack might have become more reckless, and would have been a prime candidate for death by AIDS before 1990.
One statement of Jack's that is in the screenplay, but not in the movie or novella is
If you can't fix it then, by God, drive off from it - that's my motto.
He said this on the highway, when he met Randall. Did he mean that he would walk away from Ennis, or from a tire problem?
I think Ennis will become sadder and remain withdrawn, but Jack would become possibly more "social" and less emotionally attached to anyone. Drinking would become even more of a problem for Jack.
Annie gave us two characters who cannot stand alone.
Aren't we all like that, really?