That's an excellent point. Flat-earthers as a group are an easy target because their 'belief' is essentially harmless and they're a small enough group that they really don't and can't cause any trouble; they're not, and never likely to reach, a critical mass that can alter society's momentum. Their argument also isn't compelling and cannot act as a crutch or focal point of blame for a perceived societal ill.

Racism and xenophobia on the other hand...

On democracy, I really like this quote: “Democracy is the worst form of government, except for all the others.” Democracy gives equal merit to the intelligent, the stupid, and the wilfully ignorant. This is the best we can do?

If you want a somewhat uniquely American critical-thinking-eschewing topic: Capitalism, Socialism, and Communism. That's a land mine I like to step on occasionally to gauge the current sensitivity.