The Duff CEO with a Windows-Logo on his forehead: "Gamers use Windows because of its' user experience not our de facto monopoly."
Next Image: Duff CEO with Windows-Logo in front of a "Out of Business" sign. Subtitle: "30 minutes after SteamOS is released"
Edit: Yo, I'm not saying this is gonna happen. I just want to say that Windew's UX sucks ass.
If Steam isn't a monopoly because the Epic Games store and GoG exists, then Windows isn't a monopoly because Mac and Linux exist.
Look, I like Valve. They are better than the vast majority of big game companies out there. They aren't perfect, though, and they definitely have a monopoly on online PC game distribution. We shouldn't be blind to that.
There are people using Windows who would very much rather not use Windows but need to because it is the only way to use given software. I haven't heard of anyone who would very much rather not use Steam but has to in order to access a given game.
In this regard Windows has more in common with Epic and their paid exclusives than Steam.
Which still doesn't disprove the monopoly claim. Steam can be a monopoly even if people like to use it. Valve could very well change in the future. We can hope for the best, but we're basing a lot on the continued goodwill of a single company.