As others have said, you have to define what you mean by failure. From a sales perspective, it will sell a huge number of copies... but it might be behind previous games in the series for total sales figures. Would that make it a failure?
From a gameplay standpoint, a lot of people have been happy with Gran Turismo as it is and they're only adding good things to the existing formula. But it might not have a feature or two that a competing game has. Would that make it a failure?
I would answer no to both of those questions and I'm convinced that GT5 will be a success on both fronts, even if it's not an overwhelming, blow everyone away success.