Valve is one of the less evil ones. But GOG didn’t need to be forced by court to have a no-questions-asked 30 days return policy.

Gamers can buy wherever they want. But game devs either sell on Steam or they basically don’t sell at all.
It’s not a full monopoly. He just has a quasi monopoly. And there are rumors that devs who sell cheaper on other platforms risk not being shown to buyers on Steam…
With great power comes great opportunity for abusing that power to take more middle man tax.