This ignores the existence of a world outside a single country's political system. Opinions that are labeled "extremist" by the political class in one country,

This ignores the existence of a world outside a single country's political system. Opinions that are labeled "extremist" by the political class in one country, but which are held by leaders in other countries that don't suck, are about as easy to exterminate as cockroaches. The Chinese government uses a massive Internet censorship apparatus to prevent organized challenges to its preferred ways of doing things. This has worked, but its necessity reflects the fact that key Chinese-government ideas are memetically uncompetitive on the world stage; China is severely handicapped when it comes to winning foreign hearts and minds, and it also isn't great at persuading its own talented citizens to stay when they have the means to emigrate to the West (though it is now benefiting from Western infighting). Both sides of the US political class agreed to define "extremism" in 2015-16 to encompass sensible opinions that ran contrary to the political class's common interests. However, they did not have the Chinese government's Internet censorship capabilities at the time, so there was a winning play that ran contrary to your claim, and somebody executed it effectively.