Unlike other corporations, they actually didn't really do all that much to make it a monopoly though. It's kind of an organic monopoly simply by being better than everything else, by a wide margin.

There's not much "lock-in" apart from the games one owns on the platform; and the social aspects of steam are mostly negligible or niche - sure there's the friendlist, but no gamer I know uses steam voice-chat so the friendlist is mostly replicated in discord and similar anyway.