Absofuckinglutely. Being a monopoly happens when your product is just that good. What you do when you are a monopoly, that's a different matter. And Valve is doing OK so far, yeah, not perfect, but that's how these things go.

Does this mean Steam is guaranteed to always be equally good? No.

Does this mean Steam is an evil monopoly right now? No. At the moment they are just a monopoly as people prefer them over the competition.