For me monopolies, even natural ones, are maintained due to power structures in society. If you see most natural monopolies that are privately owned there's usually a political system that allows that to occur and a well connected powerful actor. AI has the ability to break the one power structure keeping the middle class intact - that being they still technically need people to "think on the ground" to provide the things they want. i.e. output is still a function of labor. AI breaks the need for "labor" to be a required input into the production line long term - i.e. it will no longer be a large economic factor of production.

My personal view: AI makes most of the world's population in the long run surplus to requirements for people who own capital, societal power and the planet's scarce resources. That's a form of monopoly.