Valve "played capitalism so well they broke it" and now Steam can't be replaced
(www.gamesradar.com)
I don't think Steam would be considered a monopoly because they don't use anti-competitive practices. Maybe there's a technicality where the word "monopoly" fits because of their market share, sure, but they've done nothing inherently greedy or abusive in the market. They offer a great platform for gamers and game makers, and competitors have all failed to offer anything better or novel.