Japan always drops at these times, and there's little reason that the Wii should be an exception.
It is logical that Japan is harder struck for another reason too - games don't sell for that long.
In Europe,what you say doesn't seem to be a problem at all.
End May - Mario Kart Wii sells ~ 100K weekly and averages around 6th place in sales.
End October - Mario Kart Wii sells ~ 100K weekly and averages around 6th place in sales.
End May - Wii Fit sells slightly below 100K weekly, and averages around 8th place.
End October - Wii Fit sells 110-120K weekly, and is generally in the top 3.
So little games coming out seems to be no problem at all in Europe.
The same thing is true for Americas for Wii Fit/Mario Kart Wii and the other high profile games, but not to the same extent for most other games.
So the only real issue point for Wii in lack of software, is Japan - the area it sells best in compared to the competition.
Look at 2003 for the Ps2. http://vgchartz.com/hwcomps.php?cons1=PS2®1=Japan&cons2=PS2®2=Japan&cons3=PS2®3=Japan&start=37626&end=37990
Now 2007 for Wii. http://vgchartz.com/hwcomps.php?cons1=Wii®1=Japan&cons2=Wii®2=Japan&cons3=Wii®3=Japan&start=39089&end=39453
They both rise up untill March, but then the Ps2 starts to steadily drop down untill November. Wii on the other hand, keeps March levels all the time, up untill early September, when it drops to the same levels Ps2 has at that time.
If you look at Wii 08, it is more of a Ps2 03 type. Steady decline all the way untill November, where it is supposed to pick up. http://vgchartz.com/hwcomps.php?cons1=Wii®1=Japan&cons2=Wii®2=Japan&cons3=Wii®3=Japan&start=39453&end=39747
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS







