This is a great question and I love post-apocalyptic science fiction. What is potentially realistic and what is pure fantasy and entertainment? I think there are several questions that we can ask ourselves to determine what is more or less realistic. Let me give you three questions.
First, through science is the post-apocalyptic scenario possible? The follow up question is: how possible. If we can answer that it is possible, then we are treading on realistic grounds. Something like the Planet of the Apes is so far-fetched that we can say that it is pure entertainment.
Second, do we have the technology to create this post-apocalytic scenario? If we have the means to destroy ourselves, then it is in the realm of the possible. For instance, we can have a nuclear war. We have the missile and the hatred between nations.
Third, has the post-apocalytic scenario ever happened on a smaller scale before? If it has happened before in human history, then it is reasonable to assume that it can happen again.