I've thought this too but then you are leaving the future to those who breed more irresponsibly. Another way to look at it would be that we have a responsibility to raise more responsible people.
Though I agree. It's odd - I've known people who seemed offended by Idiocracy, but couldn't really come up with any flaws in its basic premise (in short, well-educated highly-productive people (for traditional values of productive which is its own debate) tend to have fewer kids, and people are usually like their parents, so over time we'll have fewer people like that).
The movie "Idiocracy" addresses this very issue. It's a comedy, and I really enjoyed it.
I think you meant prophecy.