Valve would simply refer the judge to their rather-successful competitors, including:
- Epic Game Store
- Origin
- Blizzard Store
- Microsoft Store
- GoG
- Itch.io
- Ubisoft Store
- Bethesda Store
And to further their point, Valve would show how many games on Steam are found on other stores, and Valve doesn't prevent other companies from listing games on other stores.
Valve does not have a monopoly; not by a long shot.