[Karunamon]

Actually, offline mode is a thing. You still go through the client, but the service being available is neither here nor there.

However, I find that the client offers enough on its own (the in-game overlay, mostly) that I'm more annoyed when I can't get it to work with a game. I have re purchased games I already owned on physical media and other ways to get the Steam integration (and I'm not alone there).

Steam going bust, in addition to being really unlikely, would be a mild annoyance at best, since bypassing it is a simple matter of replacing a DLL.