I don't know where this idea that Mario Party suddenly stops selling comes from but if someone look at previous entries he'll see the legs this game has. The game will be over 200k before the end of this year.
I wasn't really doubting MP legs as much as the Wii U.
I just looked at MP9 sales. Opened at ~150k and crossed 500k about 2 1/2 months later while still doing ~9k a week. Impressive.