I even doubt that people are expected to even install games onto the OS itself.
No, no-one will install games on this thing. It's not like Steam has the most successful gaming app store ever.
(I'm guessing you mean "install from a third party", and that will probably be difficult, yes.)
I expect the ultimate intention is Valve will launch a cloud service so that SteamOS is just a minimal frontend for games running somewhere else.
This doesn't make sense from a lot of perspectives. One is latency. Another is the fact that they've spent a lot of time, money and PR on making a box+OS to actually run games on, including large improvements to the Linux GPU drivers. A third is the fact that all previous attempts have failed, see OnLive.