It leaves the only moment of "swearing" or "oath making" in Ennis's life (as far as the viewer is concerned) as the moment with the shirts and the "Jack I swear..."
This is really a good observation. Another possibility, another layer of those two words.
The guilty look on Jack's face at causing Ennis's nose to bleed seems like an interesting parallel to the guilt that Ennis seems to carry with him about that awful punch. It always makes me happy that he brings the punch issue up in the motel (you can tell that it's just been eating at Ennis and he was probably slightly worried that something like that might have prevented Jack from contacting him for so long... or it might have fed into his anxiousness before the reunion kiss when he says to Alma "if he shows"). It's cute that in the motel Jack doesn't seem all that worried about that issue... he just lets it go and doesn't respond to that comment.
I bet he kicked himself a million times for that punch, starting with riding down the mountain, when Ennis already looks guilty, then beside the sheep pen, then at the truck, then in the alley, and then constantly for the next four years. When he says "I thought you were still sore from that punch," he sounds so casual, but you just know from the fact that he mentions it at all that he has always feared his rash violence ruined his chances of seeing Jack again. (The other confirmation being his exact quote of Jack's in, "And the Army didn't get you?" showing that he's been going over and over that conversation for four years.)
You are both so right. Poor Ennis (and poor Jack for that matter). I always felt it would have needed only a little bit to change the situation at their last day. Usually I want to shout at Ennis during many scenes of the movie. But when they actually part, at Jack's truck, it's Jack I want to shout at. I have the feeling a little "Ennis" or "Wait a minute", or something along that content from Jack could have made another scenario. One of these "What ifs..."
These "What might have beens", the "What ifs..." and the "If onlys..." are part of the magic of BBM. They keep my mind running in circles, envisioning different scenarios over and over.
Back on topic:
I've thought about this two words many, many times, as we all have. I can think of many possibilities, which all are valid. And they have been already mentioned on this thread.
For me, the fact that he
is finally swearing to Jack is the most important. He
is making a commitment, and it is all the more tragic that it comes too late.
And for me, it means: Jack, I swear, if I had another chance, I would make it different. I would make better descicions and I would live with you. This implies naturally the "I love you and always have", but goes beyond it.