What devs would like to do is push gamers to buy their games on platforms where they get a higher revenue cut. The logic is that 15 dollar game could be 12 dollars on GoG and a dev makes the same amount of money. The issue with this approach is the game theory doesn’t work out for the customer - if a customer is willing to buy a game on steam for more money, than the dev has no reason to offer it on another store for less money when they actually have a higher cut of the revenue, and a higher incentive to price higher. There are non-price reasons a customer would like to buy on steam. Cloud syncing, integrity verification, easy steam deck client, and most importantly parity with the rest of their library. So it is very favorable to Valve as long as its the preferred place to buy games.
The issue is that while it is fair for steam to compete on service, it definitely is unfair for them to try to dictate to devs and publishers where they can sell their games and how they price them, especially if it isn’t a part of their ToS. You don’t get a steam key from GoG, and you aren’t using Steam’s servers. If Devs get more of the revenue, AND they were theoretically able to achieve more volume on GoG, why would you ever sell your game on steam and give up more revenue? Steam has to compete - which they claim they do, and David is alleging they don’t in their suit.