If anything he risks footing a lot of the effort to get developers onto Linux (see: Valve openly talking about working with GPU manufacturers to improve the quality of Linux drivers) only to have it benefit people that choose to distribute outside of Steam.
No, because it's DRM-less.
That said, Valve appears to be going for a play based on being a cross-platform app store (Windows, OS X, Linux). It's not obvious to me that they can add much value that way (after you've gone to the work to implement support for three platforms, submitting to three app stores is not a lot of overhead) but we'll see.
I know that Steam lets you take your games from one machine to another, but I don't expect that to be the norm for most app stores.