"Monopoly" is a distraction. The issue is abuse of market power. Having market power is fine. You can't punish people for being successful.
Steam doesn't abuse being successful to lock out competitors. You can sell products sold through Steam via other platforms too. You can sell outside of Steam and give your customers Steam keys for the game. You can install Steam on different platforms alongside other stores and programs.
Nothing Steam does makes it harder for consumers to buy games from Valve's competitors. That's what matters, not whether Steam is very successful.
To be clear, I don't think Valve has abused their position at all. I was merely musing on how they could. Which would operate on a similar concept as Apple did: "my users will stay in my ecosystem almost regardless of what I do."
Steam doesn't abuse being successful to lock out competitors. You can sell products sold through Steam via other platforms too. You can sell outside of Steam and give your customers Steam keys for the game. You can install Steam on different platforms alongside other stores and programs.
Nothing Steam does makes it harder for consumers to buy games from Valve's competitors. That's what matters, not whether Steam is very successful.