Dusk: Unpopular opinion: I'd rather pay Valve 30% and put up with their de facto monopoly than help Epic work towards their own (very obviously desired) monopoly
Monopoly doesn't mean "Largest market share". It's a real term with a real meaning.
Monopoly:
the exclusive possession or control of the supply of or trade in a commodity or service.
What, exactly, does Valve control? They don't require exclusivity, they don't require their DRM, they don't require the use of their network system. Hell, they don't even require you to to give them 30% if you sell your own key.
Valve is also not a publicly traded company, while this doesn't mean you can fully trust them it does mean they aren't required to seek profit at all costs. This allows then to do things like, support Linux, make their own hardware (twice after their first attempt was a failure), work on Proton, develope games that make them no money, etc.
Itch.io, GOG, EA, Epic, Windows Store, Game Pass, Humble Bundle, personal websites. These are all examples of places you can buy video games on computers.
Timmy Tencent's propaganda is working on you if you think Valve is any sort of monopoly.
Courts do not require a literal monopoly before applying rules for single firm conduct; that term is used as shorthand for a firm with significant and durable market power — that is, the long term ability to raise price or exclude competitors. That is how that term is used here: a "monopolist" is a firm with significant and durable market power.
I don’t think Steam qualifies still. There are still plenty of competitors such as GOG, Green Man Gaming, itch.io, Epic, Humble Store, Microsoft Store, and so on.
The "significant durable market power" part is why I went on to explain how they don't lock you into their ecosystem. How can Valve raise prices or exclude their competitors when they literally do not have any mechanisms in place to do any of those things?