I think the 8th gen is a localised low point for console sales. I expect Switch to do better than Wii U and 3DS combined. I expect MS to not screw up the launch of XboxTwo and so they will do better, and perhaps bring some PC gamers back into the fold (I don't think PS4 gamers will convert in substantial numbers). And I expect Sony to have taken note of its PS3 struggle, its PS4 success and Xb one's low performance and therefore do everything possible to emulate their PS2/4 experience.
I think there will be modest growth from the 8th Gen only. Sales won't get anywhere close to the 7th gen. And I expect console sales to level off in the long term, neither growing a lot or shrinking a lot.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."
Jimi Hendrix
