Without a doubt yes. They already do for the most part. Steam sales are the goal of the industry, thats why epic is having to go to the lengths that it is to try (and fail) to get customers.
steam already:
- restricts sale prices off platform
- limits what a publisher/dev can have as a discount price
- limits when a publisher/dev can change their price
- restricts access to free keys for games
- dictates the standard for revenue sharing
- forces steam to always be at least equal to the cheapest price around
- restricts putting an item on sale outside of the platform unless there is a planned sale on the steam page in the near future
Like I can say for certainty yes, due to even a handful of these restrictions, if steam decided to unilaterally apply an additional base fee of x% of the game cost (which they can do), devs would be forced to either abandon steam (again the largest PC gaming market out there) or raise every other storefront price.
There will be other options yes, but it would be like opening a lemonade stand in a dark alley vs at a busy crosswalk. Steam would need to raise the price significantly in order to convince a studio is who trying to make a profit to jump ship.