You're using a different definition of monopoly from what I'm using. To quote Wikipedia:

In economics, a monopoly is a single seller. In law, a monopoly is a business entity that has significant market power, that is, the power to charge overly high prices, which is associated with unfair price raises.

I'm using the latter of those definitions. I don't think it's particularly useful to only consider it a monopoly when there are literally no competitors. I think it is useful to consider it a monopoly when it has dominant market power. Steam's estimated 75-80% market share is dominant market power.