MS was anything but lol, they had a swarm of lawsuit dramas over by then. Only Apple glazed MS because they quite literally saved them from bankruptcy which they only did as insurance against a proper antitrust case.

Google was newer but they also weren't a private company, they were riding silicon valley money to the moon.

Regardless, Steam is a monopoly but their immediate competitors are the only ones chucking anti competitve measures around like crappy DRM and price lockins.

Other gaming platforms are fully capable of competing with Steam. It's only real edge is that it treats game developers better than any other platform.