A great thread, this. Really interesting to see the 2015/16/17 battle and this may just top it, so thank you OP.
I didn't think 2018 was going to be its peak year. However, this was before the 2017 Holiday performance came fully into view. There's definitely potential there. The way I see it is it could go either way, I think 2018 will be absolute minimum of 18.5M with the shape of releases this year, so it won't drop much, if it does.
However, there's definite scope for a relatively bigger increase. I thought 2017 was on for 21M, nigh-on 22M. I still think that's achievable in 2018. The games are there, the missed holiday potential from 2017 is there for 2018. However, it's looking like a staged price drop leads to matching the potential, culminating in a $199 for the entirety of the 2018 Holiday period. There's a far better chance for Sony to feel comfortable with that price for more than just Black Friday and consumers haven't been exposed to it for a sufficient time that it wouldn't bump sales up. Hell, their Black Friday price might just be lower than it was this year, if they make $199 permanent.
Like I say, very easily possible for it to go both ways, but based on the first week, I'm going to be optimistic and say 2018 will be the peak.