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Not having a roadmap doesn't really matter, indeed. A roadmap is just some very vague schedule that show when some things may be done.
Having no plan or goals like Thom said, however, is far more severe. A plan is like a strategy. When a certain amount of complexity has been reached (both KDE and Gnome *are* complex), a project needs people who coordinate it, whether you like it or not.
So if there really is no goal or plan set for the future I believe Gnome is a sinking ship. I doubt however, that that really is the case. For example, they still have their philosophy and HIG to give them at least some direction.
And while it may indeed be true that Gnome has no concord innovations on the roadmap like KDE has, I believe that those will appear bottom-up, and be given some structure by the HIG and by their philosophy. Its success however, will largely depend on the people who are at the helm of the project.