Because there's a certain subset here on HN that believe just about any problem could be solved if only the free market was allowed to operate without restriction or government intervention. I have on occasion run into them like running into a brick wall.

It's the opposite. You don't need to regulate the free market itself. You have to properly regulate society itself so that the free market can actually align incentives with desired results.

Government intervention can be fully compatible with the free market if the government acts as a market participant.

High school econ class, for one

I know many people who hold onto the idea of free market like it's a religion. They call their religion libertarianism (I don't know if this is accurate of libertarian thought leaders)

I do have to refute that idea. I'm not sure how much their opinions matter and if my time is well spent trying to change them, but they're in my life so I collect the evidence.

To a first approximation, zero Libertarians believe that "the free market can solve all problems." Many of us do believe that - on balance - free markets deliver better outcomes than other approaches, but that is not a claim of perfection. Other Libertarians believe that free market approaches are simply morally superior, since any non-free-market approach seems to necessitate the use of force/violence to prevent people from simply trading freely as they would in the absence of said force/violence.

And, of course, some Libertarians hold both beliefs - that free markets are the moral approach AND that the create better outcomes more often than not. But other than maybe a few n00bs who just discovered Libertarianism from a pamphlet or something, almost nobody is going around claiming that free markets fix everything.

Since no purely free market system has every existed at the scale of a nation in human history and therefore no coercion free one either at what point does it become acceptable to arrange your society around coercive arrangements.

If you construe some tipping point like for example chance of existential or utilitarian failure that does justify coercion how do you differentiate between a society in which minimizing coercion is the goal and one in which whatever you construe as justifying coercion is actually the guiding principal.