And then you said:
People insisting that Steam is a monopoly are implicitly arguing action should be taken about it.
So I guess the question is - what do you want to do about it? If you are in fact saying it is a monopoly (and you're not just saying you say it's a monopoly) and saying it's a monopoly must necessarily imply a call to action: what is that action?
The right answer is that there is no such call. So maybe you could stop insisting there is, and about ninety percent of this conversation would fall under 'sure, oops, nevermind.'
Meanwhile any conversation about Steam's marketshare - for example an article asking "Can anyone overthrow Steam on PC?" - kinda has to bring this up anyway. It has an impact... and a lot of people will implicitly or explicitly say nuh-uh, at length.