Kevin Kelly of Wired Magazine believes that no human invention ever goes extinct. National Public Radio has recently had some fun with this, challenging its listeners to identify an invention that the world has stopped producing. It turns out that Kelly may well be right; finding something that absolutely no one makes any more is a daunting task. You can learn more about the debate here.
I suspect that Kelly's point is just part of a broader one: no idea ever goes extinct. Justice Holmes's familiar metaphor maintains that ideas compete for allegiance in the marketplace of free expression. Yes, but it is important to remember that this process does not cast the losers into exile. It just relegates them to the darker corners, where they can continue to serve the customers who still patronize them.
So do not feign astonishment when obnoxious doctrines, long thought extinct, reappear. No idea ever goes out of production. No idea, no matter how pernicious, ever falls entirely from use.