> It is not at all proven that a free market will naturally devolve into a monopoly.

I think it comes down to the category of item being offered in the market. Some things naturally lend themselves to monopolies; it feels like perhaps it's based on factors like the difficulty of entering the market with a new product at all, the amount of coordination and manpower required to field it, and the cost efficiencies of having a singular producer vs. many.

There are certainly cases where we see duopolies or triopolies etc. where one really-well-run company might be more efficient, from a labour standpoint; but then, in turn, we all benefit from having a redundant array of supply chains.

There are other cases where we absolutely want a monopoly, such as with policing, or (in many countries) with healthcare, because they apply to everyone and being a consumer of the service is not exactly optional.