There are only two possibilities; One, she has been murdered and the body hidden...two, she has been abducted and held out of the sight of the public. As horrible as they both seem, i hope it is the latter. There have been quite a few recent incidences where a child has been kidnapped and kept hidden for a number of years, but they have escaped and their life goes on...albeit with the trauma of the event.
It also shows us something important about policing. There are some crimes that, for whatever reason, aren't solvable. It also shows us how important the "Golden Hour" is. The Spanish police failed to appreciate the importance of the golden hour, and it could have been their incompetent that sealed Madeleine's fate.
Its right that British bobbies keep this investigation open and active. I believe that this crime is solvable. Someone somewhere has seen her, and the Met police are going to have to be the guys to find that person.