I wouldn't say you own games on Steam. You can't install them without Steam, and usually you can't even launch them without being connected to Steam. And you if lose your account, they are lost forever.

GOG on the other hand, you can absolutely download offline installers for you entire game collection and keep them somewhere safe in a hard drive from which you could still install them 10 years after GOG is dead. (and hopefully the games still work on newer hardware)