I dont expect MK8 to sell more either, but i have to say that Mario U wasnt just given for free. Thats a misconception some people have. The Mario/party bundle is 2k yen more expensive and the Wii fit is 4k yen more expensive. Since those are digital games, its very possible that Nintendo is gaining money from it (specially since its not like mario would sell much more).
Next month Nintendo is going to release another sku with a new remocon and Wii sports and that one will be given for free, since it will be 30k yen. In Mario case, its like anyy new copie of it is 1000 yen.
And about MK8. No way its going to sell more than FF15 unless final fantasy really underperform in the long run and MK have really nice legs. Not that i think its going to be a bad seller, but lets have some reasonable expectations. Just because 3D world outperformed Lightning returns does not make MK8 > FF15 a possibilty.