Short Answer : NOSTEAM, I'll just leave it at that...
Longer Answer : In the unlikely event that the Valve/Steam monstrosity dies, as GrootHammer pointed out the companies IP/Assets will be sold off to the highest bidder. This will most likely ensure that the steam service persists in some way shape or form. Failing that it will be a matter of time before someone brings out a local spoofing server or a community based server authentication method to allow you to use titles in your current state. Also titles may be made available though other channels depending on what happens with the sale of assets and IP.
Officially Valve have stated that " they would patch the games to not require steam to run anymore so you could download them and back them up "