No matter the reason, private monopolies are a bad thing for consumers.
The game devs and publishers set the price by taking into consideration that 30% goes to Valve, without that 30% games would be cheaper as they wouldn’t need to sell for as high a price for the devs and publishers to recover their investment.
No need to have studied economics to understand that if you need to have 30$/copy in your pockets in order to cover your cost and someone takes 30% from every sales then you need to sell to the consumers for 43$.
No matter how nice Valve acts towards consumers (in many cases because it was imposed to them, not by choice), in the end you’re defending a billionaire while you make less a year than he spends running one of his yachts for a single day.