"You don't get my point. Wii-fit isn't fun. Brain Training is. Here is the difference."
When Brain Training came out a lot of people complained that it was a non-fun non-game (at least in the west, don't know about Japan) but it still sold a lot because while a lot of people (including many hardcore gamers) don't find it fun there are enough other people that did find it fun enough to warrant spending money on it and nothing about you thinking Wii Fit will not be fun prevents other people to think that it is fun.
Whether there are enough people in Japan that think it is fun in order to make it a success there is debatable for the next few weeks/months (months if it opens to low sales but has big legs, like Brain Training) but your opinion of it does not prevent it from being such a success.
Another example is many people think JRPGs are repetitive "press the A button to win the battle" grindfest but it doesn't prevent many of them to be very successful and it doesn't make them any less fun for those of us that do like them.
In conclusion, don't say "this game is gonna bomb because I don't like it" but say "I don't think enough people will like this game to make it a success", ideally with a solid reasoning why you might think that is the case.
"I do not suffer from insanity, I enjoy every minute of it"
