Valve "played capitalism so well they broke it" and now Steam can't be replaced
(www.gamesradar.com)
Monopoly: exclusive ownership through legal privilege, command of supply, or concerted action
Steam has legitimate competitors. They do not have exclusive control. They are, however, dominant because of the incompetence of their competitors. And there are absolutely issues and risks that go along with that. It is not a healthy market, but insulting people when they correct you about the monopoly label is not helpful or productive.