It's hard to really call Valve a monopoly when, there is competition. If there's no competition, then Valve would clearly be a monopoly.
It's not like back in the 90s when Microsoft bundled their Windows OS with Internet Explorer that edged out Netscape back then. Because there really wasn't a lot of browser alternatives available to have made it where competition was there. Microsoft was considered a monopoly back then because competition was very little during their peak then.
In the digital PC gaming landscape, it's entirely different. There are numerous marketplaces for digital games. And they're big enough to where Valve is just simply an alternative and can go without if someone chooses.
Valve doesn't force anyone to use Steam or strong-arms people to buy games from them. They just exist, the people have spoken both by their own loyalty and their wallets. And that made companies like Epic mad and jealous. They just came late into the game when Valve was developing itself.
I've played a lot of ARPGs and every one of them has their different flavors. But, I'm a type that wants a specific flavor and trying other flavors just makes me feel like it's not good enough. It's kind of like it's a discount version of that flavor I like. I'm playing a game that's inspired by the game that defined the genre. Just to expand a little, I am not trying to say that the games mentioned are terrible. I've touched upon Grim Dawn and Titan a few times.
But a lot of the time, my mood and feeling always darts for Diablo.