Be careful with that 'marketplace of ideas' metaphor

In many discussions that swirled around the Internet in the wake of Brendan Eich's resignation from Mozilla Corp., someone invariably invoked the concept of the "marketplace of ideas"—just as in the market for physical goods or services, ideas themselves are brought to the "marketplace" of discussion, weighed on their merits, and adopted or discarded as this "market" dictates. If you hang positive traits like "freedom," "openness," and "meritocracy" on the market-metaphor, that's all the more reason to apply it to many different spheres.

But it's worth noting some possibly uncomfortable implications.
Even a cursory glance at any industry, and the history thereof, shows that not all goods exist in the market, that were once there. That is, sometimes it's simply unprofitable to try to sell things at any given time or place. There's not too much of a market for horse-drawn carriages now, for example, or Tandy 1000 computers. Products are obsoleted, discredited, go out of fashion, and so on, even before government regulation (or industry collusion) enters the picture.

Hence, properly extending the market-metaphor to ideas, it should be a matter of course that some ideas just deserve to die, as the market dictates. And yet, unlike "real" marketplaces, there's nothing quite like the pervasive belief that not only does one have the right to voice one's opinion, but that somehow the opinion deserves to be heard. Or given space to breathe, safe from criticism.

It would be like a shop owner demanding that passers-by give him money, or a CEO demanding that the stock market reward her company just because it exists.

Maybe this difference comes from the fact that anyone can hold and voice an opinion (be a "producer" in the marketplace of ideas) but not everyone is a producer or seller of "real" goods. And yet in simulated environments where this is possible (online role-playing games, for example) we don't see that much of it. Maybe the obvious unreality has something to do with it.

Or maybe this is where the metaphor breaks down, and we really should treat ideas as fundamentally different from "real" goods.

I'm somewhat split about this. On the one hand, I do think that it's good to dredge up old ideas, re-examine them, and re-criticize them if necessary. As products of the human mind they can provide valuable insight into its workings, as well as predict future trends in human society and thought. But that's not so different from, say, an engineer keeping old models to reference when building a new and improved one. I think it's more that ideas can be so powerful that we shouldn't cast them into oblivion (or adhere to them blindly) as the popular will might entice us to.

On the other hand, I think that some ideas deserve to be quarantined, as it were, inside a cage of permanent skepticism and ridicule. We must not forget the old bigotries and intolerances, superstitions and pseudosciences, because they have a nasty tendency to recur in subtly new forms. There should come a point where some ideas are so thoroughly discredited and beaten that to seriously consider them is to seriously mark yourself as ignorant or backward-thinking.

On that point, I think that traditional-marriage-defenders are on very shaky ground.