An interesting thread!
Erm...he is the best in the world by far and you could say that if he loses no more than 5 matches per year for the next 3 or 4 years then I suppose fans could lose interest in the game because there is no variety in the tournament winners.
HOWEVER, I don't stand by that point of view. He is clearly a breath of fresh air and has raised the standard of the game to a level which the other top players will match within a few years.
Besides, he isn't unbeatable - he is most vulnerable on clay and so there will always be times when there are surprise winners.
Also, he has dominated for the last 12 months, maybe fatigue will still be there by the middle of January - a loss at the Aussie Open isn't unquestionable.