As with most interview questions, the "correct" answer varies from interviewer to interviewer. However, I think the general idea is to express that (1) you learn from your mistakes, and (2) you're committed to medicine. So, you say something about asking schools about your weaknesses and doing whatever was necessary to fix your weaknesses and then reapplying (I think just saying, "I'd reapply" indicates stubborness but not really adaptability). You also mention whatever (research, community service, work, academics, etc.), you'd do in the meantime.
If you want to cut off the question, you just say, "I've already received an acceptance, so it's a moot point."
In terms of those who actually don't get in anywhere and give up on going into medicine, they go do what the 99% of the population who aren't doctors do-- get a non-physician job. Go to graduate school, work in consulting, teach, etc.