Selling quicker doesn't matter much - see Wii. PS2 sold in good numbers 5 years after its peak - it's that tail that pushed it out furthest.
If PS4 nose-dives like PS1 or Wii, it won't make the numbers. All we know for sure is PS4 is on the downturn. I can't find a more recent graph that this Ars one which doesn't show PS4's latest decline.
Thanks for this, that’s a great chart
The big question will be how much PS4 price can drop...it’s all very easy to say selling quicker doesn’t mean much, but it sold quicker at a much higher price against much stiffer competition.