This argument could be made much simpler by observing that centrism is simply the middle of the road fallacy turned into an ideology. As the middle of the road fallacy is unsound by definition, any positions taken on the basis of such an argument are liable to have nothing to do with reality, and any decisions made by such an argument are likely to have unintended or harmful consequences.

Of course, some people will also hide behind this argument because they want to use certain extremes as strawmen so that they can use centrism as a smokescreen to hide the true toxicity of their real beliefs that they want to push. In these cases, the middle of the road fallacy will often be accompanied by many other fallacies as well.

In any case, it should be sufficient to point out the fallacious/illogical nature of their ideology and arguments to show that these people should not be listened to or taken seriously at all. (It isn't sufficient in practice, because most people are too far removed from reality/epistemological soundness to be saved, but it should be. It will be for anyone with a functioning brain in their heads.)