I don't think it would. DirectX is the only thing that stops games migrating to other platforms. The steam front end is just a gui, it can be rewritten for linux (and probably already exists on linux in the value studio). Games themselves don't care what platform there on provided they have access to the correct resources, again directX. PC games have been developed previously on DOS, windows mac and linux. If blizzard can make all there games run on mac and windows with minimal tweaking from the same CD, then getting a tweak to make windows games run on a linux based OS won't be much harder for valve. The problem is all games currently ask for there resources through directX, This makes things impossible for companies wanting to extend away from MS as they have to go back to basics and deal with an API. Simply copying DX's link library would warrant action from MS.
As for not hurting windows.
Free linux with full game support, I'd have it on every machine i own. I would never buy another copy of windows again as too would a huge number of PC gamers. Linux is just a better OS in terms of stability, and resource management since you can do what you want with it. And most programs have a linux variant, just like macs do.