I work for a community college and pull / grab / design data for various audiences - the state, the county, the faculty and the powers that be at the institution.
I think everything is going to fail this generation relative to the previous one.
Wii (100-105m) --> Wii U (50-60m)
PS3 (90-95m) --> PS4 (50-60m)
X360 (85-90m) --> Onebox (60-70m)
I figure:
PS3 (11m Japan, 27m US, 33m EU, 21m elsewhere) ---> PS4 (6m Japan, 16m US, 26m EU, 8m elsewhere)
Wii (13m Japan, 42m US, 34m EU, 14m elsewhere) ---> Wii U (8m Japan, 22m US, 16m EU, 7m elsewhere)
X360 (2m Japan, 45m US, 25m EU, 16m elsewhere) ---> One Box (1m Japan, 35m US, 20m EU, 8m elsewhere)
I think social tolerance for gaming is about 10 years behind in the US vs. Japan, the PS1 era was the console peak there (19m PS1s, 11m N64s & Saturns - 30m consoles total, PS2 era was 28m (w/ DC, or 26m w/o DC), the current Wii era will be 26m - so you can make the case that with no population growth gaming shrunk in Japan each gen by 2m from the PS1 --> PS2 --> Wii eras. SNES + Turbografx + Gen did about 23m in Japan, so we're sort of moving back toward the pre-peak era figures.
The US market in contrast has gone like this:
SNES + GEN - 40m
PS1 + N64 + SAT - 47m
PS2 + XBX + GC + DC - 76m
X360 + Wii + PS3 - 114m
...but the last two years have been pretty similar to 1999-2000, which is unusual - usually the transitions take you back one cycle, not two. But the numbers have been like late in the PS1 era. So I think 2002-2009 may have been the largest the US video game industry will ever get...which leads to this:
One Box + Wii U + PS4 - 73m