Enforce interoperability, lower Valve cut as abusive and punish abusive clauses in developer agreement (you can’t price your game lower than Steam on other storefronts).
Ideally you’d treat Valve like a telecom monopoly, meaning you’d have to break it down into two companies - Valve infra (handling license ledger, storage, bandwidth) and Valve store/developer. Allow other stores to notify that user owns a game and allow access to Valve infra with third party stores. Valve infra can’t give preferential treatment to Valve store.
If Valve is as good as everyone says they have nothing to fear, they provide better service after all. Where I live this approach killed monopolies and prices dropped quickly.