I think Steam is an anomaly, not the rule for monopolies. Steam is privately owned with long term stable leadership. They are generating a crazy amount of money and are able to be content with that.
If steam went public and had the usual revolving door of MBA CEOs keen to "maximize efficiencies", you can bet that Steam would turn just as malign as the adtech industry.
I concur on all points, though I think there's something else than public ownership at fault per se. Publicly traded corporations were once considered an innovation and improvement over private ownership. Something went awry over the years, and private equity is presently giving a bad name to private ownership too.
Private equity is a type of private ownership, even if it doesn't apply to Steam. In the broader context of business dysfunction, public-vs-private ownership is not telling the full story. Corporate raiding is also just part of the picture; MBA-driven corporate mismanagement, ZIRP and LIRP, principal-agent mismatch, short-term profit maximalism, and a number of other issues are involved too.
A private founder-owner-CEO combination (like Steam) might be the least vulnerable organization to many of those ills, short of a cooperative (not sure it counts as private ownership).
If steam went public and had the usual revolving door of MBA CEOs keen to "maximize efficiencies", you can bet that Steam would turn just as malign as the adtech industry.