Well the last game to do 500k+ in a single release was December of 2005. The best version was able to sell another 130k, with 80k of those in 2008. Destiny from November 2006 was able to do 500k+ with the Director's cut. This was after the first DS game sold around 200k. There doesn't seem to be much series decline (besides the incredible numbers done by Destiny/2). The GBA games actually did worse than the DS/PSP ones, so there isn't really any decline there. The 360 obviously isn't going to sell 500k, so that pretty much leaves the Wii and the PS3. There has certainly been some mismanagement of this series. Namely that they haven't been able to handle the transition to portables as well as some other games. I'd still say it has the ability to sell 500k.
Anyways, my main point is that calling it niche is just silly. That would be like calling left 4 dead niche in the west.
