It's not the graphic increase but the little device it comes with that is going to be the sales driver. Go look up the game. It's kind of intriguing. Certainly not the level the first game was but it should sell well.
The issue is that if we consider the majority of wii fit owners are housewives, they don't give a toss about a pedometer because the wii fit still works for them, and no pedometer is worth $350+.
Think about it this way - you don't upgrade dumbbells.