In light of the inevitable comparisons to the Mac App Store, this shouldn’t even worry Apple a little bit. Steam does have a large built-in audience, this is true, but it still isn’t an “app store”. Apps distributed via Steam won’t have access to a lot of great MAS-only features that Apple device users want: iCloud, Notification Center, etc. Plus, there’s just a certain level of confidence that comes from purchasing apps in the App Store; while most geeks might be more security conscience and they find the App Store policies restrictive, it’s still nice to know that Apple is actively working against keeping dangerous software out of the store. Additionally, there is an artificial ecosystem of quality growing up around the App Store the leads to a need for differentiation and strong design/developer effort if you want to be competitive in the app store.
This might be great for apps that don’t need MAS features to open themselves up to a larger audience, or apps that don’t meet the sandboxing requirements, but if anyone needs to be worried about Steam distributing apps it’s gonna be Microsoft. I can see the competition in this space having a much larger impact on the Windows Store than the Mac App Store.