I don't think it will drop to under 10k, but it sure dropped horrendously YOY. Check out the Wii YOY figures:
2008 2009
75,824 26,000
I think it will level off at middle tens, it would be horrible if it dropped under 10k, since it would become a 'dog product' in the eyes of Japanese consumers.
Final Fantasy: Echoes of Time is coming up this week, and Monster Hunter G in April, so that should improve Wii hardware figures.