SimonG: And let's be honest, this is something that no retail game can offer, so who would have had demanded it when DD was coming to the market.
SirPrimalform: It's not exactly as if installing a game causes your computer to go "Om nom nom nom!" and leave you with no disc, is it? :P
They could just give you the installer for a limited dowload time. After that, you better take care of that installer (like you have to take care of your disc). The whole "account package" that GOG offers is certainly not something you would have expected 10 years ago. The irony is that it was probably because Steam used its client as DRM, they "had" to offer this service. Otherwise it would have been really ridiculous, an always online DRM (yeah, yeah, offline mode...) but no option of a digital backup, who would do that (apart from Ubisoft).
If the first major DD would have had GOGs model, who knows if they would have been inclined to offer the "internet backup". Or they would have probably demanded a small fee/extra "warranty" or some of that BS.
GOG is "the best of both worlds", well, actually it is "better than both worlds", as you don't need the installer in a special folder to play the game.