One thing i want to point out is that each time a Nintendo console declined gen over gen ws due to a different reason.
The main reason for SNES dropping from NES was due to increased competition from Sega in America. The Genesis had a two year headstart on the SNES and was also able to release a killer app in Sonic before SNES released. Add to that the effective marketing campaign labeling Nintendo as "kiddy", which Nintendo kind of helped them out by censoring Mortal Kombat. Overall the drop in sales from NES to SNES was just under 13 million with America accounting for 10 million of that.
The reason for N64 numbers declining from SNES levels was mostly due to Japan. If you look at the top 20 SNES games in Japan it consists of 9 RPG, 6 platformers, 3 fighting, 1 racing and 1 puzzle. When Nintendo chose cartridges over CD for N64 they lost support from many third party developers. RPGs like Final Fantasy, Dragon Quest, Chrono, Mana were all now on Playstation and Saturn had alot of smaller RPGs as well, also the Street Fighter fanbase had moved onto 3D fighting like Tekken and Virtua Fighter which were on competitors consoles. Basically N64 was just a Nintendo box now. Overall the drop from SNES to N64 was 16 million with about 12 million happening in Japan alone.
The reason for Gamecube dropping from N64 levels was again mostly due to America. Goldeneye was a huge hit on N64 in America due to its revolutionary split screen multiplayer selling nearly 6 million and becoming the 2nd best seller in the region only behind Mario 64. When the Xbox released along with Halo it essentially stole the FPS market from Nintendo leading to Gamecube having a harsh drop in America. Overall the difference between Gamecube and N64 sales was about 12 million with 8 million coming from America.
So to recap SNES sold less than NES because they allowed Sega to cappitalize on a 2 year headstart in America, N64 saw a drop from from SNES because they lost many big third party franchises in Japan, and Gamecube did worse than N64 by losing the FPS crowd to Xbox in America.
When the herd loses its way, the shepard must kill the bull that leads them astray.
