The only thing that requires Steam to still be around is the multiplayer system Steamworks.
In order to play *any* game bought from Steam, the Steam client must be running and have an internet connection.
That's not true! I have several games on Steam that will run just fine on it's own (I can even copy them to my laptop without Steam installed and they work there too).
Not all the games on steam use Steamworks or any sort of DRM...
Kerbal Space Program is one example.