Casual crowd really inflated last gen's numbers. Since they moved on to somewhere else, there will be an obvious decline.
And Vita won't get anywhere near PSP numbers, so you also have that. But when you only consider PS4, One and 3DS, I think the numbers will be, proportionally, not very far from 7th gen. I honestly do not expect a massive decline or the death of the consoles (unless there's a drastic change of paradigm next gen).
Edit: And just for shits and giggles, here are my predictions:
3DS - 112 million
Vita - 41 million
PS4 - 107 million
One - 72,2 million
Wii U - 37 million
Total: 369,2 million
I can see someone saying I'm a little too optmistic, but I think it's plausible enough.








