You really gotta aim your sights higher if that's the criteria you're using for a "monopoly". Valve is a private company, that sells games and other "wants", not "needs". If people can't afford games, without losing their house or struggling to eat, I don't think that's a company's fault.
If Valve was even close to using anti-competitive methods to maintain market dominance, you'd be correct. However, a company having superior quality products and making good business decisions is not a basis or definition of a monopoly. They just make good decisions and provide quality products that people want and enjoy.
Instead of using strawman and false equivalency fallacies, try taking a look at what really constitutes anti-competitive practices.