I certianly don't see why it is the only way they can go. There is no reason they can't write original stories or even base them off of Bond books written by people other than Fleming. In fact once they finish Quantum of Solace, they will only have about one or two more original Fleming titles at their desposal that have not already been made into a film. As much as I would love to see them make a version of Live and Let Die or Dr. No that are more true to their source material (as much as Casino Royale was true to its source anyway), I just don't see them "remaking" any of the ones that have already been used for a film title. Every major Fleming book has had a film named after it so far, with Quantum Solace they grabbed the title from one of his shorter stories, which some of the old films did also.

Just looked up what original Fleming titles are remaining, we have two stories in "For Your Eyes Only" titled "Risico" and "The Hildebrand Rarity", and we have a story in "Octopussy" titled "Property of a Lady". None of those sound like movie quality titles. Perhaps Quantum of Solace is the last Bond movie to be blessed with an original Fleming title. Anyway, I had hoped they would disregard the previous films and go ahead and continue Casno Royale with an adaption of Live and Let Die, which was the original sequel to Casino Royale.