Wii fit was massive, but part of that success was built up demand for the wii itself. I purchased a wii/Wii-fit... even my mother in law got one... but don't see either us or them getting a wii-u this time around.
This time, because there is alot of apathy about the wii-u and the larger "casual" gamers are not interested in it, i dont see people rushing out to buy Wii-u Fit. If they market it well (alot) i think it would help sales to wii-u..but I really don't think it's going to be a game changer. I really do think the wii-u is pretty much low 35 million console sales lifetime and i am not sure anything is going to change that.