Player has choices, players picking their favorite store.
In what way is that monopoly?
Microsoft has a really bad rep about their store. They had a Windows Store GFWL years ago which they shut down, removing all games people have purchased.
I dont mind using their current store, but I understand why people won't.
Epic Launcher launched with a really lackluster store. I bought one game on their store, World War Z. After they handed it out for free later, I felt like I wasted money on it and dont buy anything there anymore.
I occasionally buy on GOG, or, its a while since at least, because I am one of those booting Steam up.
But, Steam being popular doesnt make a monopoly. Monopoly is theres no other place to buy your games. There is.
You can even buy steam keys from third party sites. So you can even buy games from other places than Steam, and activate them on Steam.