If they bring their A game.... they could probably manage that.
I think 2017 is the PS4's peak.
The question is how much over 17.5m is it going to sell this year? I think its over 18.5m.
Necro-bump this 2020: http://gamrconnect.vgchartz.com/thread.php?id=229249
Bumb in 2021: http://gamrconnect.vgchartz.com/post.php?id=9047071