I think this year will be the peak year, so ~18M sold through. We have GT Sport launching, which is Sony's largest exclusive. And we also have a $199 PS4, at least for the holidays, and a $349 Pro.
Next year, I think we will see sales start to slip as the gen starts to age. I'm guessing a full $199 PS4 and a $299 Pro will help it not be down by too much, so possibly ~16M sold through. And in 2019, we will have a few exclusives help it not drop too much, again, so maybe ~12M-14M. At this point, I think Sony is going to sense the writing on the wall and launch the PS5 late 2019, coupled with a cut to $149 for the PS4. The next year should be down, again. So, ~8M-10M. In 2021-2020, I think we are going to see the last few sales of the PS4. So, ~6M-8M for both.
So, in the end, it'll probably sell ~115M-120M. Possibly more.