If Ennis would have gotten into the truck and they'd ridden off together, nobody would have made a movie about their lives. We'd never have heard of them, and wouldn't know to be glad that things had worked out happily.
So if there were to be a happy ending, I'd prefer it would happen at the end of the movie as we now know it. Ennis would start to drive away after the lakeside argument, then after half a mile or so he'd go, "Oh, f*ck this," and do a U-turn back to the lake. That could actually still be a movie! (Maybe not quite as good of one, though ...)
As for The Graduate, my understanding is that that sort of bewildered silence at the end is supposed to leave us thinking, OK, so now what? Benjamin and Elaine go off and get married and start a family and buy a house in the suburbs and ... become plastic, too. That probably wouldn't happen to Jack and Ennis, though.