> The marketplace of ideas only works that way when its participants are rational actors with good judgement
You're leaving out the implicit assumption you're making, that the "authorities" in charge of the marketplace are rational actors. You better hope to hell, that they are.
Because when some marketplace participants fail, they fail gracefully. When centralized authorities fail, they fail spectacularly and disastrously. I can virtually guarantee you that Qanon will be a footnote in ten years time. In contrast how would you like to live in Iran, China, Turkey, or any other regime where lunatics have taken over in the name of "protecting the people from dangerous ideas"?
All those people clamoring for authorities to clamp down on irrational Trumpist extremists, have forgotten that Trump and his cronies were just recently in charge of all three branches of the government as well as a supermajority of state governments.
This is how the political cycle always goes. All anybody sees is a sensible, incoming administration with likeminded views. And I actually agree. Biden's a decent man, and I honestly can't see him abusing even very broad powers. But what happens after? But there's no putting this genie back in the bottle. Do you trust President Donald Trump Jr. or President Tucker Carlson or President Josh Hawley with that power?
This isn't even hypothetical. Most people here are too young to remember the post-9/11 hysteria. But I can virtually guarantee you that had the current crop of anti-speech activists managed to put their would-be "anti-extremist" laws in place, that opposition to the War on Terror would have been de facto illegal.
Dealing with some stupid conspiracy theories and LARPer shamans, who have as much chance of overthrowing the American government as my toddler, are a small price to pay for fault-tolerance in liberal democracy.