One of the themes or insights of the story is into the fallability of humans. Not only do roles get reversed and Larry ends up as the drunkard rather than his father, but it is also a story where the father swears off drinking, but of course falls prey to the danger again later in life. O'Connor makes it clear that humans are not really going to end up in many "happily ever after" stories as their weaknesses remain weaknesses.
Another theme is that of the helplessness of humans to help each other in certain situations. Because of the way the story turns out, the reader is reminded of the times they've watched others fall whether it be to drink or some other weakness or vice and felt helpless to intervene or to help.
Even the small victory that Larry and his mother have at watching his father so disgusted by his son's behavior which was mimicking his own serves to remind the reader of the fact that human weakness is a constant.