Agree with most you say. Just two things
Steam doesn't own the market place, as said, gog and itch.io do their stuff, epic is also there (nobody with a brain likes them but they still have a share) and then a publisher could just make a website for their game, Minecraft for example.
They don't decide either, the algorithms within steam work very clearly and their seasonal sales are from my knollage open sign up for the devs and publishers. The player specific feeds also work according to tags, play a lot of builder games recently? Steam recommends similar games you might like, sometimes mixes between tags you haven't played like that.
In reality it's almost exclusively up to the devs/publishers and the players what gets sold steam does push indie stuff a little more in recent years but I don't see the downside of that.
And secondly.
"Leftist" real left people would be happy that steam is how it is and would bring constructive criticism. The people screaming Steam bad, are the same people that scream everything else when they get cloud from it.