Publishing is a massive undertaking. Albeit Steam is big, if you're going to make AAA games you need to sell multi-platform and have a definite retail presence (The only reason why games like Metal Gear solid and Uncharted done come to the 360 & PC is because Sony heavily subsidize them in order to keep them platform exclusive). I just don't see Valve wanting to get into that space, especially with the next console generation likely 6 months to a year away in terms of announcements.
In addition, they have consumed development studios before; granted, they're not exactly large ones when they do, but they could definitely handle extra talent if they saw it fit. Whilst it is unlikely, I wouldn't say put Valve out of the running entirely for getting a group such as Relic.