Crosmando: Digital distribution is the way of the future, if anyone distributor is going to go out of business first it's the old brick-and-mortar retailers like Gamespot/EB Games etc. Plus GOG was the "first" (not counting DotEmu which isn't well known) DD site is offer games completely DRM-free, and DRM-free is definitely the way of the future too with more and more gamers every day demanding the reduction (of eventual end) of DRM on games. So I would say as long as games continue to be made, GOG's future is assured. That doesn't mean they have a license to slack off getting all the old games here though...
gameon: The thing is though, anyone can add games and do the drm free thing. There are many competitors, and you must have seen companies in the past do anything it takes to beat their competition. Perhaps GOG and Steam will be the equivalent of ATI and Nvidia in terms of market share. It could also be possible that GOG may lose out to Steam, Gamersgate, Dotemu, Origin, Desura, Green man gaming, the list of competitors goes on.
Of course, which is why GOG always needs something to distinguish itself from potential competitors. Steam has the client with integrated multiplayer support for Valve games, plus integrated community and workshop support for modding.
GOG does have the "old" games thing to distinguish itself, no other site really has the old DOS games, also GOG could do well by promoting itself as selling "old-school" games like retro-ish indie games or just games that don't fit into the AAA-game mold. If this stuff on Kickstarter with DFA, Wasteland 2, Project Eternity turns out to be a real resurgence in old-style games, GOG could do well by riding the wave of this new market.