I'm pretty sure that the Steam store is going to go through significant changes with the release of the Steamboxes. I think it's going to be quite a lot harder to be profiled on the front page. Hopefully that front page will be curated by knowledgeable people who are good at spotting quality games deserving space there, but that might be a little too optimistic given how crap Valve were at finding quality games back before they did Greenlight et.c.
Bottom line is that pretty much every PC game will be on Steam, but only some games will actually get any decent attention there. It's good news for those who know how to dig and likes to have everything they play on Steam, and good news for a lot of developers, but it will also ensure that "being on Steam" isn't going to sell games by itself any more (I think that for the most part we've passed that particular point already). So PR is going to be even more important than it is now.
Whatever happens, Steam won't die.
HypersomniacLive: First off, they need the consent of devs/ pubs to patch them into a non_Steam_DRM-ed state - what happens if devs/ pubs don't consent? What if the games are deemed too old by devs/ pubs to spend time, money and resources to rework them into some other type of DRM?
Steam does have an offline mode, and most games work offline. So it's unlikely that it'll be a big deal for Valve to set Steam to be offline "forever", should the need arise.