Have you considered maybe Valves monopoly is natural?
Yes that changes literally nothing.
All monopolies, be they natural or otherwise, need to be heavily regulated or else they can:
a) easily do stuff to prevent competition. Stuff like preventing developers from selling their game for cheaper on other stores.
b) charge exorbitant markups, markups like 30% of all revenue for a listing in a store.
I do not understand why gamers have such a hard time grasping that Valve taking a massive cut off the top of every single game sold, just enriches the already rich for doing nothing, at the expense of consumers and creators.