Steam

requiring a conviction as a monopolist as the only acceptable form of evidence of monopolization?

Legally speaking, yes.

Ethically speaking, I have not seen sufficient evidence to call Valve a monopoly yet. Obviously everyone can call Valve a monopoly, or not, I don't really care either way. Actual evidence would have to come in the form of documents proofing Valve manipulates prices, hinders competition or anything similar.

Unfortunately most documents in the Wolfire vs Valve case are not publicly available. The point Wolfire makes in their statement about not being able to sell their keys cheaper than on Steam, has some merit but I will leave it to the judge to decide on that one. It's not enough for me personally to call that anti-competitive.

If someone said the same thing about Google when Epic sued them in 2020, would you have waited the 3 years it took to get a trial verdict before making up your mind?

I did make up my mind, but there was no evidence until the ruling.

Also, many arbitration settlements include NDAs as a condition of getting a payout, so it’s disingenuous to say they could provide evidence that might require their clients to forfeit their settlements or risk them getting disbarred.

Fair point, many do. They could mention it though.

but why are you insisting on giving the giant for-profit corporation the benefit of the doubt rather than the consumers who are trying to hold them accountable?

Because I am the consumer in this case and I don't see any wrongdoing by Valve in this case. There are other store fronts on PC, Valve doesn't force any prices, they don't force exclusivity, they don't buy competition up and they don't prevent the competing stores from functioning in any way.

People simply flock to Steam because it's the best service and until Valve engages in (proven) anti-competitive behaviour, there is no reason to change anything about that.

Is the 30% cut they demand too much? Yes. Are they engaging in unethical gambling? Yes. Are they a monopoly? Not in my opinion.