In that case, it would be a slight plus even. Having a big player actually contributing code often is a win-win. Plus, I just stumbled accross Steams own announcement where they explicitly state that people can install their own launchers and games.
But it seems like they could block access to other launchers or executables, say by signing or encrypting their own Steam executables, as their DRM does for some games already, and refusing to launch anything unsigned, but then people would likely just go back to vanilla wine and spread the word, which would be bad for everyone.

@FarkyTheDog: If you're referring to the sad state that Galaxy development seems to be in, then adding another version to maintain would certainly not be helping, agreed. However, I wonder if the lacking MP facility in some games isn't directly related to Galaxy missing, because frankly, Linux has always been very good at two things: file systems and networking, so there's no technical reason why it wouldn't work on Linux while it does on Windows and even Mac.