It is a monopoly.

Semi-monopoly, not a true one like Apples store on the iPhone. Also no one is forced to use Steam, there are alternative shops. The PC platform is open. In fact the most successful games aren't even on Steam. If Steam was a real monopoly, then this wouldn't be possible. If you purchase a XBOX handheld that is based on PC hardware, you don't have to install Steam, you can just use the XBOX shop. If you want you can install GOG and Epic shops too. Maybe the League of Legends launcher too.

And it goes a step further, because game keys for Steam can be purchased on other platforms and shops without paying Steam a single cut of the price. And yet, those games purchased with such a key are integrated into Steam and Valve provides "free" service to them. With all having said this, I don't think one can Steam call a true monopoly.

Their 30% cut is egregious.

But its industry standard, not really egregious in the sense of abusing the market. Valve also provide services beyond just marketing their games on the platform, they host the games, provide features, do research and make it better and so on. Also the cut gets lower to 25% and then 20% if it sells better (which is not helping indie devs at all). Having all said this, I think 30% as the industry standard is way too high, but its not a fault of Steam only.