PC Master Race

Specifically, they request that any game that is listed on Steam cannot be listed somewhere else for a lower regular price.

Officially this only applies to Steam Keys. Unofficial there are a couple of allegations that they've done this in other contexts. This is not "numerous testimonials proving" nor is it reflective of actual policy. Part of the legal process is determining if the allegations are factual and if they represent actual policy or if they're the actions of individual managers misunderstanding or overstepping policy.

If the allegations are true, yes it's monopolistic behavior. In the meantime you're free to use Epic, Origin, Ubisoft Connect, GOG, itch.io, whatever Microsoft is calling their store these days, Humble, or wherever else people publish their games.

Valve makes it easy to buy and sell, simple to update, and has integrated social features like a friends list and messaging that simply work. They keep margins low by having a small number of employees. They don't need to engage in anti-competitive price bullying to be embarrassingly profitable. Maybe they are anyway. I'm not going to use a worse store for no gain while I wait for those lawsuits to finish. (I do like to buy through GOG or itch.io when it's an option because I'd rather have the software unreliant on a third-party or internet-connected launcher)