I guess I will repost my chart because you guys are still arguing about this:
Notes: 09's release is the only home console on this list (Wii).
I believe all numbers are single SKU.
This set of numbers comes from Capcom's official data for all games over 1mil sales.
Slope is around 0.3
Notes: This is a chart of first week sales in Japan only
Data comes from VGC
Slope is about 0.16
As you can see, theres really no validation to the claim that Monster Hunter is going downhill, at least with the data we have. It seems to me like it has peaked and now it has stabilized and the sales are certainly more than healthy. Make of it what you will, but the phrasing "downhill" doesn't really fit