I fear you will be right.... The casuals will eat it up.
I think alot of kids will like it. I remember being little and wishing I could play an instument. It will go over VERY well for the 3-10 year old demographic. I also think women may like it. So 10 million is not out of the question... and don't forget its biggest strengh... It is a Wii series game.
End of 2009 Predictions (Set, January 1st 2009)
Wii- 72 million 3rd Year Peak, better slate of releases
360- 37 million Should trend down slightly after 3rd year peak
PS3- 29 million Sales should pick up next year, 3rd year peak and price cut