They have a near-monopoly on PC gaming, which sounds bad at face value. But the only reason for that is that they've continually developed an excellent platform that helps connect people with games that they want to play, and then be able to play them. There are very few marketplaces that strike the right balance of presenting you with options you want, while not artificially getting in your way. (In other words, it's not very enshittified.)
There's nothing stopping someone from never buying another game on Steam, and moving to another marketplace on PC, unlike the store monopolies on consoles and mobile devices.