Right, I am sticking my neck out here. I have created a chart which demonstrates what I reckon will be the absolute maximums for all 3 next gen consoles.
As you can see by the chart, I reckon the absolute maximums for each console to be:
Wii - 85m, PS3 - 78m, 360 - 40m
I may hear a few chuckles around, but I don't think all the consoles will reach their absolute maximums. Although the Wii is the most likely out of the 3.
The total absolute maximums equate to over 200m worldwide. Currently the last gen consoles sold around 180m worldwide, and I have taken into account the sales of the DS and PSP taking sales away from the home consoles.
Realistically, I see the Wii being between 50-85m lifetime; the PS3 between 40-78m and the 360 between 25-40m lifetime. If I was to stick my neck out further, I would say.....70m for Wii, 60m for PS3 and 30m for 360 which would make a worldwide total of 160m in the next gen's lifetime.
Prediction (June 12th 2017)
Permanent pricedrop for both PS4 Slim and PS4 Pro in October.
PS4 Slim $249 (October 2017)
PS4 Pro $349 (October 2017)