As far as I know there is no mandatory DRM on Steam either, so if a publisher wants to they can just make their game be portable and not require Steam to even be installed. Pretty sure all the re-releases that use DOSBox or ScummVM are like this, for example.
Steam's new disclaimer reminds everyone that you don't actually own your games, GOG moves in for the killshot: Its offline installers 'cannot be taken away from you' (www.pcgamer.com) angielski