Nobody should be paying those prices though.
Steam, PSN, and XBOX stores all work because there are alternatives for customers to shop at for a fair price. If you don't like the prices on Steam's main page you can always just buy a Steam Key from another web site at a fairer price. If you don't like the prices on PSN or XBOX you can always just find a physical copy for less.
I agree, or at least I wouldn't pay those prices anyway.
"But what can a Stadia customer do if they don't like the prices listed?" - Very soon there'll be heavy competition with cloud gaming services. XCloud, Stadia, PSNow... we know EA are working on a cloud service too. Everyone's going to be trying to get in the ring and there will be price wars. Just not in the next few months.
Stadia may essentially have a monopoly at the moment, but that will change quickly. Then the competition for game prices will start and will be fierce. With Zero barrier for entry (No expensive hardware to buy) these services will have to offer the best price and best performance for their games or people will just buy it on another service. There'll be no problem switching between them on a whim.