Hmm... that's a good question. Just as you've done, OP, I'm gonna try to break this down console by console. I'll also try to be generally optimistic with my estimates, after all, I don't want another decline :(
PS3: With it being discontinued in Japan soon, and the overall trajectory going massively downwards, I'd say reaching 200K should really be the goal here.
X360: Considering it's already been discontinued, shops are likely to be just flogging the last of their stock and when it's gone, it's gone. Probably 100K.
Wii U: Once again, it's already been discontinued, so it's a similar story to the 360. In this case though, I'd suggest 200K.
PS Vita: The Vita's been going down for a few years now, and with the introduction of the Switch, that decline could well speed up. I think that Japan could see a a drop to about 400K but Europe should hold better at 600K, and then with the rest of the world (inc. the US), the Vita should be able to manage 1.2 million.
3DS: Last year's 7 million is really nothing to go by. With the Pokemon GO craze followed by the launch of Sun and Moon, the sales last year can't be used as a reliable reference point. Looking at this year, we can see that there are basically no new games coming (aside from FE: Echoes and MHXX in Japan, both of which aren't system sellers because any fans already have a 3DS). Also, the Switch is the 3DS's replacement, meaning anyone who wants a Nintendo portable is probably gonna look there. I'm gonna be generous and say 3 million.
XB1: I have to say that Microsoft have really gotten off to a slow start this year, which certainly won't help them reach a peak year. Looking forward, the exclusives list for Xbox, especially without Scalebound, is looking waaaaay too thin. I don't think the Scorpio will have much of an impact just like the PS4 Pro didn't. I'll say 7 million.
PS4: Despite a phenomenal start to year, the PS4 reaching a peak year may be harder than some think. Last year there were two hardware revisions, VR, and fantastic system sellers like FFXV and Uncharted 4, yet it still performed with slightly less sales than in 2015. Having said this, the PS4 does have a lot of momentum from the start of the year, and the relative weakness of the Xbox One, in addition to a slew of system sellers to come this year. I think that the PS4 is probably gonna hit 18 million.
NSW: Now this is the wild card. I know that the system has gotten off to a great start but from here on out, there are gonna be a lot of droughts with only Splatoon in the summer and Super Mario Odyssey in the holidays looking to be real system sellers. I think 8 million is probably a sane bet, but I could certainly see it going over that with luck on Nintendo's side.
So then, what's the total? About 37.7 million. So yeah, I think that this year will see a very slight increase in hardware sales. Going forward, I expect to see console sales stabilise at between 35-40 million a year. Hopefully that's enough to be able to sustain the market :/
