Steam isn't blocking anyone from competing as far as I'm aware. It's just that most customers don't to switch to a new platform because none of the competition is better than they are and we don't want to have to juggle multiple launchers. If Steam started being assholes to the customers or developers stopped putting games on there we'd look at other options.

Supposing Steam is a monopoly what remedy do propose that wouldn't make the user experience worse for their customers?