Gambling / trading aside, they're doing a pretty fine so far.

Other developers can just make their own platform or distribute games themselves individually. Nobody wants to do that for obvious reasons.

There's clearly a network effect similar to Instagram, WhatsApp, Facebook and the likes. But apart from regulating for service interoperability, there's nothing you can do.

It's like claiming that Skype had a monopoly between 2008 and 2014. Just... Install something else then?

Valve is not buying other store makers nor actively sabotaging efforts to do so.