There have been good open-world games with good stories, and there have been open-world games with terrific side quests - random stories and such - but not with quality main story lines. I've yet to play an open-world game with the branching narrative style that CDPR uses, one with huge impact on the game. How will they pull that off in open world?
I think you're getting a bit too stuck on that open-world thing. Open-world doesn't necessarily mean a Skyrim style RPG, especially since what's been said about the game so far is mostly PR hype that doesn't really reflect what the game will really be like.
Plus, you can definitely do a branching storyline in an open-world, if you link the branches to different factions, like Fallout:New Vegas does.