For sure, valid to fear the enshittification of steam. But they aren't killing proton. Maybe ignoring proton at worst. But Steam has profit motivations for not being reliant on Windows, which has actively been trying to supplant them with the Windows Store for years.

As another separate, profit-motivated company, with a gaming division and a lot to gain from eating Steam's lunch, Microsoft is not Steam's friend. Proton is a critical bargaining tool for them, and not having to include windows licenses for devices like the Steam Deck helps their costs too.