Steam is definitely not literally a monopoly. Most people don't seem to know either of the most important things about monopolies, which is 1) what one is and 2) that it's not necessarily relevant whether they are, because antitrust doesn't require a monopoly. It only means you're abusing a somehow dominant position in a market.
With that said, I've been using Steam for a lot of years. I've had many technical problems with it, but I'm not aware of any way in which they are abusing their position. I haven't exactly been looking for this information either, though, and I do see that the article mentions a lawsuit I seem to remember something about.
I think 30% is a lot, but I don't think Steam is really doing anything to prevent anyone from releasing anything else anywhere. They also allow a whole lot of content that is frankly surprising for a mainstream game site, they are ultra-liberal about permitting whatever and I think that's pretty amazing. I do worry about the whole thing packing up one day, but that's a whole other discussion. Are there really any complaints about Valve that have legs?