I don't know what the Steam api looks like for developers, but--I guess I don't really believe that this is some accidental bug. The default behavior should be that everything sans multiplayer still works without Steam. Obviously this is possible to do--because some games do it--but it's also obviously not the default behavior, because so few games do it. That benefits Valve--how does it benefit players?

As I mentioned, I have already lost the ability to play certain games on legacy operating systems because Steam dropped support, so I don't consider this merely a theoretical concern.